1) IPS has been warning users of this for months. IPB 3 is a complete rewrite from the ground up utilizing new, more efficient PHP 5 code. The base code is completely different, and it's a totally new version. You simply can't expect to move forward this much and be able to use old modifications. It's irrational.
2) I disagree that there are "drastic" changes in navigation. Some people obviously dislike change, but I think the layout, location/placement of features, navigation, etc. are much more intuitive and user-friendly.
3) Same answer as number one. As the software code is brand new, so is the skin and templating. IPS has been warning users of this for months, during which time you could have sought out a designer to upgrade your skin to IPB 3 or make a new one for you. With a new major version and major improvements in such a version, it is inevitable that old, outdated skins will no longer work. Same goes for modifications.
4) If you want access to support, upgrades, and Resources, then renew your license for $25 for six months. You can't expect to receive all of that for free. How could the company possibly earn a profit?
I think it's fairly insulting to say that "IPS discards past customers as being a valid part of the community" when without a doubt that is not the case.