Pokémon with an exclusive Ability that serves to just changes it’s in-battle forms tend to not stood the test of time either. While that worked on Mimikyu and Aegislash, there is no denying that they had gone overboard to the point questions are asked if all those form-changing Abilities are really worth it.Aside from evolutions and regional forms, moves and abilities also help Pokémon stand the test of time. Which is why I hate how much signature features newer Pokémon receive. Gen 7 is the biggest offender of this because almost every, if not every evolutionary line received a signature move, ability, or both. This is especially annoying when new Pokémon steal signature features of old Pokémon while simultaneously receiving something exclusive for themselves. Lunala stole Night Daze, Calyrex and Mr. Rime line stole the Ice/Psychic type, Togedemaru stole Iron Barbs, Rolycoly stole Heatproof, Magearna stole Shift Gear and Heart Swap. When people were speculating a Z-Move for Magearna I wanted to facepalm because it already has Fleur Cannon and Soul-Heart. It does not need any more exclusivity.
What's particularly annoying is how some newer signature moves eventually gets distributed to older Pokémon over time like Solar Blade or Psychic Fangs. But to promote newer Pokémon, they wait an entire game or generation before giving newer moves to older Pokémon. And even then some moves like Floral Healing or Zing Zap are still exclusive or not distributed enough. Sadly, abilities do not have the same luxury since they change a feature of a Pokémon instead of adding onto what's already there. Hidden abilities were a great way to introduce newer abilities to older Pokémon, but I don't think they plan to add another hidden ability slot. Some abilities like Triage or Merciless deserve to be given to the original forms of older Pokémon. Triage Mega Audino is such a waste of potential, but I rather have it on its base form (or both). Now I don't mind some features being exclusive to certain Pokémon, but later generations have too much special treatment.
And to add onto DrPumpkinz's post about items and the general discussion on why Z-Moves and megas do not help Pokémon stand the test of time is how they stopped making regular but exclusive items like Thick Club. Regular items are a more permanent feature, but since Gamefreak doesn't want regular items like Soul Dew outclassing whatever gimmick they want to do with the item slot, they stopped making them. In fact, Thick Club is the only great signature item for a fully evolved Pokémon that's not a Z-Crystal or mega stone. The rest are either meh (like the nerfed Soul Dew or Creation Trio orbs) or for unevolved Pokémon (Light Ball or Clampearl evolutionary items).
I still stand about the fact that several single-staged Pokémon introduced in generation 3 aged the worst, but the other single-stages relying a lot on their gimmicky Ability, except Mimikyu, Minior and Wishiwashi, can and will age poorly.