Yes, this is totally off-topic. And its wrong, imho:This answer could seem Off Topic but the programmers developing the game will understand it (I hope).
In the early 1995, more than 25 years ago, I was developing a program for an insurance company.
It was always growing with customer requests, so I was using relational databases, just not to stop the customer using the program and giving the possibility to add more records/databases to the new requests that the new model of insurance policy was requiring. My customer is still using the same program I did that time because it's better managing the payment form I personalized for the customer's situation.
The point is this: you can't "grow" the database otherwise you've to restart programming again stopping the customer using it? Easy. Just add more database "related" with one record (son) to the database (father) and you'll get the relationship you need without going to change, anytime, your program disappointing your "customers" they'll have to reinitialize their "work" over and over again.
That was happening 25 years ago... and my program doesn't need any update from me, even if the customer add 50.000 new records. This is why "cannot use old saves with a new version" get me (and not only me) sad.
25 years ago, it was impossible to compilate sorgents who was ending in executable more larger than the dimension of a floppy disk (1,4Mb) so we was solving the problem with the overlays... today, it's not a problem anymore (thank God).
Have a good time for the modders and the beta-testers, anyway.
*Sorry for my bad english. I'm trying to improve it anyway.
There is data (relational database, save games, game assets).
And there is behavior (database client app, game engine and game logic)
If your behavior changes, there is a chance that your data is now obsolete, or being used in a different way and must be restructured or altered in some way or another.
If you worked with relational databases in the past: this is why people use database schema migration tools.
A game developer could spend time and energy on making save games forward compatible by migrating old save games to the newest format. But that would be a huge liability that would slow down the development, and it could introduce its own bugs. So it is a tradeoff, and as long as a game is not released I can totally understand why the developer choses NOT to spend time on forward compatibility of save games.