Other gen2 mons of gym value

Double Fighting Ursaring may have value in the same role as machamp. Kingdra, depending on moveset, can fill some roles (i.e. double dragon vs. Vaporeon).

50% meta shift is due to Blissey (the new and better Snorlax) and Tyranitar (the new and worse Rhydon)
50% meta shift is due to moves rebalance

Other gen 2? They are just there for fun, not for fight. (could be used to prestige though)

Old gen1 mons with good moves like Machamp are going to stay much more relevant.

Donpham and Heracross?

Umbreon is great for prestiging. It is very tanky, but with low cp compared to its performance. I feel like it's similar to pre-nerfed Wigglytuff.