I can see how you might look at it that way in terms of saving money. But we spent a lot of time, resources, and money in development of those 3 years of features and other clients have an expectation from us.
The only way to make self-hosted viable for the future is to ensure people are paying for the effort put into developing it. Otherwise we really should just get rid of it. If you are not willing to do that, there are cheap or low cost alternatives out there but you don't get the owners of the company replying to you and you most certainly do not get monthly updates and constant improvements. Look at literally any other community platform and you will see many are at worst dead and at best barely supported.
It is also very, very common. I'm not sure where you have been living in 30+ years of the software world. It's very normal for software to require a subscription fee or to require a re-purchase at major versions. I get emails all the time from software I own that says "new version pay up!"
We are not requiring people to re-purchase for v5 as other platforms would. The "catch up" renewals only apply to the monthly option as it is basically a finance version of the annual fee. You may wish to review the new terms in the client area as you seem incorrect here.
We are growing and, if you are actually serious about your community, a few more dollars a year to get a platform that is actually alive must be worth it. If having something that is actively developed is not worth it to you, then your community will probably suffer with another platform.
But, as has been said a lot already in this topic, you don't have to do anything. You can stick right where you are on your current terms.
TL;DR: We are committed to our clients using Classic, self-hosted who are committed to their communities being a success. It is a joint venture.
And to your examples:
1. Insurance companies actually do charge you more if you have a huge gap in coverage. They see you as a greater risk. Same idea here.
2. Microsoft does not do upgrade fees they just make you buy the new version outright. If you would prefer that, I would be HAPPY to do that as it would make our software cost way more.