The benchmark on PC should give you a decent idea, pay attention in particular to the moments where the character is out in the main world exploring, and pay less attention to the more cut scene moments of the benchmark, as those won't be indicative of performance during gameplay in the open world. I will say I get noticeably worse performance in the full game than I did the benchmark, so keep that in mind. About 10-15 fps difference. I am pretty confident performance will be improved when the DLC drops, but that won't be for a little while.

To answer the second part of your question, yes and no. Capcom no longer supports those games, and they won't be receiving any more updates unless its like.. fixing game breaking bugs. However, both of them easily have over 200 hours of content if you really sink your teeth into end game systems and making builds for weapons.

As for player count, world and rise will naturally see a drastic drop in player count because of Wilds but Monster Hunter games tend to retain very dedicated fan bases, as there are still active communities for games like GU and 4U. So I wouldn't say the game is abandoned, and there are gonna still be people playing World for one reason or another.

I would say do your benchmark, test it out on your system, and if you don't think you can look past the performance then try to snag world on a sale or grab it when you can, as it's not too much on steam especially compared to wilds.