You work for a company for 25 years and only get paid once?
Just kidding. I'm just playing with what my weird mind thought the first time I read what you wrote.
Anyway, you acknowledge that your software did have revisions from DOS, to multiple Windows versions, and now 2 iterations of web based software. That is what the OP was complaining about.
Now its great that your software company provides all these for free. That is your choice. Avid on the other hand, chooses to charge for their updates which I suppose is their choice. It is neither wrong nor unwise to do so. To not provide updates, that would be unwise. Which is my point above.
Don't stop making upgrades for Protools. Avid may charge but keep it reasonable (unless Avid is as generous as your software company and wants to provide it for free for the next 25 years or more). I wonder if shareholders would approve that.