DawPi

Yes, but the alternative would be to uninstall and have data loss. 
I figured the best solution is to leave everything in place so that when you create your application to replace the plugin, your installation can transfer those settings, etc to your app.

Custom applications will remain untouched. That includes files and data. For development environments, you would upgrade to v5 and then start with updating your files. It was mentioned above that this might be impractical, but the reality is that every file must be modified regardless; at the very least, the class needs to be renamed to remove the underscore.
In a live environment, your application would be disabled and locked until a new version is uploaded. Note that we did this when upgrading to 4.5 as well.
Plugins will be deleted completely, as the entire table is dropped. Your local files should still be present, though. 

Thanks, but the video is 720p blurry when entering fullscreen YouTube.

Only add? What about modify/replace/adjust/update/remove?
To be clear, we can't do anything from this list, right? I mean that list:

So you're saying the initial topic could for a particular forum have a dedicated field for data entry? Would this field data be displayed inline in the body of the post after submission or would the field value be a column for the forum (searchable/filterable)
IE Technical forum you could have a discrete field "Software Version"  - will the field also include all the various types of fields like in other areas: 

Considered and decided against it faster than a quantum computer could have managed it.

Publicly released. Are you aware of private, custom mods? 🙂

That will be covered in the next blog entry.

Great! Now we need to experience it in practice. 👌

To be clear - inside system/ directory we would have access to Content and Member folders only? What if we want to adjust/modify Output/Sitemap/Helpers/Lang etc. classes and methods inside them?

Sooo maybeeee you could add BIG RED TRIGGER to enable hooks with all the risk and consequences ceded to the owner of the forum?
It would be an excellent compromise.

Maybe. Someone say yes, other no. I have plenty private custom mods (hooks) and I can take responsible and all consequences. Now it won't be possible.
~ 85-90% I think.
I prefer to wait for more dev blog entries before I'll be more critical, but I feel close to certainty that more than 80% of my projects will not be possible on v5.
 
Nope, disagree. Hooks are powerfull.

I'm just curious, @Daniel F as far I remember you wrote that your SEO app will be ported into v5. But how? I can count all hooks used by this app and there are ~ 15 file hooks. Many of them are hooked into "not allowed by v5" areas. So how? 🙂

I was just curious to know if most people just use the base IPS with the official apps and don't care about using third party apps/plugins.
That is indeed an advantage. And third party developers will also save some trouble because there is less risk of new IPS version breaking their apps.
If it's something that needed to be done, i guess we will all survive like it happened in the past with other major changes.

Maybe. Someone say yes, other no. I have plenty private custom mods (hooks) and I can take responsible and all consequences. Now it won't be possible.
~ 85-90% I think.
I prefer to wait for more dev blog entries before I'll be more critical, but I feel close to certainty that more than 80% of my projects will not be possible on v5.
 
Nope, disagree. Hooks are powerfull.

Not every person has applications/plugins associated with the Marketplace. Many people downloaded the resource in xml/tar format and just installed it.
There are definitely more installations. IMHO.

So regarding this assumed 80% of plugins being lost, the main questions I have are:
Can similar results (i.e. a client wants X added/enhanced to any part of the suite) be achieved with whatever IPS is developing for v5?  Maybe in a different way, but the same end result/solution for a client? If so, will the solutions be Marketplace sellable assets for developers? If not, how does IPS plan on addressing this clear gap in providing functionality to clients that Marketplace devs previously filled in v4? I can't imagine IPS removing the ability for devs to enhance the IPS suite in ways that IPS isn't willing to invest in doing themselves.  That would be detrimental to all parties involved.  And history (per the number of Marketplace assets) shows there are A LOT of assets/resources clients want and use that aren't part of the suite.

Maybe. Someone say yes, other no. I have plenty private custom mods (hooks) and I can take responsible and all consequences. Now it won't be possible.
~ 85-90% I think.
I prefer to wait for more dev blog entries before I'll be more critical, but I feel close to certainty that more than 80% of my projects will not be possible on v5.
 
Nope, disagree. Hooks are powerfull.

Sure. Basically, when we think Daniel, we think llamas.

Very nice inside jokes. Maybe someone spotlight it a little?

Maybe. Someone say yes, other no. I have plenty private custom mods (hooks) and I can take responsible and all consequences. Now it won't be possible.
~ 85-90% I think.
I prefer to wait for more dev blog entries before I'll be more critical, but I feel close to certainty that more than 80% of my projects will not be possible on v5.
 
Nope, disagree. Hooks are powerfull.

No issues there. He won't be able to upgrade 2/3 of them. Lots of time saved! 😂
 
Seriously, I have several apps and plugins extending classes in the /system folder. That means they're all dead for v5. So far, I don't like the direction v5 is taking at all. No matter what awesome tools you may have in place, being unable to overload (almost) all classes like before is a huge issue.
 
Well, that's it for me for now. I won't say anything further until there is more info about those "awesome tools" you mentioned. But I don't have much hope for them.

Maybe. Someone say yes, other no. I have plenty private custom mods (hooks) and I can take responsible and all consequences. Now it won't be possible.
~ 85-90% I think.
I prefer to wait for more dev blog entries before I'll be more critical, but I feel close to certainty that more than 80% of my projects will not be possible on v5.
 
Nope, disagree. Hooks are powerfull.

Maybe. Someone say yes, other no. I have plenty private custom mods (hooks) and I can take responsible and all consequences. Now it won't be possible.
~ 85-90% I think.
I prefer to wait for more dev blog entries before I'll be more critical, but I feel close to certainty that more than 80% of my projects will not be possible on v5.
 
Nope, disagree. Hooks are powerfull.

Matt, could you clarify exactly what you mean with “less”. As far as we can see, there won’t be ways to use hooks anymore. Is that right? I don’t see the Hooks tab in dev center.
What you’re calling listeners that is fired in specific events can’t be called hooks at all, so I guess that’s the reason for such confusion. At least to me. 
Once and for all, will we be able to hook in class?
Thank you.