I've just finished Undertale (both normal and true ending), and I don't really think that it's "one of the best RPGs of all time", but it sure is a good game.
I think that it's always very hard to know directly how a game will be remembered. Some games were considered as mediocre when they came out (like Demon's Soul, Nier, Rez, etc.) and are now considered as highly valuable hidden gems, and some games were considered as a messiah and finally got pretty forgotten afterward...
Except from some "instant classic", I think it's difficult to predict how much influence a game will have on future games...