What if the tx become incompatible with future versions of Bitcoin? In which cases it would just stop being compatible? If there is no compatibility guarantee, how can this be useful at all?
It should be, since all changes to the Bitcoin protocol should be agreed on by the entire community. However, there is no way to guarantee a softfork can't be abused, or even necessarily invalidate such a transaction, and there has recently been hardfork efforts by incompetent people to promote breaking compatibility, so I would advise against relying on it at this time.
It's not that simple. Even after a new version is released, breaking backwards compatibility, it has to be adopted. It would have to be decided by the bitcoin community to go ahead and run the new version of the software.
This happened with ethereum. An exploit occurred on a smart contract and a new version was released where the hacked funds couldn't be touched. They effectively just removed the capability in the software.
It's kind of scary when you think about how a developer can modify a file and tamper with your money, but doing so is a major action not taken lightly.
The ethereum blockchain is still split. The one with the hacked coins is worth about 100 million dollars and the one where the hacked coins are frozen is worth about 1 billion dollars. I would imagine you would see something similar to that at the least if such a change were made to the software that prevented access to funds.
Ultimately it comes down to whether the change is legitimate or not. If it's legitimately being removed, then the developers could easily gracefully remove it. For example disable the feature so the funds are available immediately.