Is it safe to keep ONLY a time-locked tx? 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.