Among the various ideas people have had for balancing gyms, a common one is each gym should not allow duplicate pokemon of the same species. As much as I too dislike seeing the same pokemon recurring in gyms, I don't really like this idea. However, it could work really nicely to restrict legendary pokemon like that. Maybe even go as far as only allow one or two legendary pokemon total in a gym for future generation's sake so that gyms don't just become all legendary.
Generation 1 legendary Pokémon
Do you think when generation 1 legendary Pokémon get released, will they replace the top tier Pokémon currently now.
Answers
Maybe, I see Zapdos being the most pain in the ass to deal with since Moltres and Articuno are weak to a lot of things compared to zapdos but for sure mewtwo will be top. Mew we'll see how it plays out. I would be fine with gyms having no duplicates in the gyms just so it won't be pure legendaries in the gyms though with more generations coming the more legendaries are added and we'll most likely see gyms with nothing but legendaries either way so I guess thats a problem to deal with later when it comes which will start when the gen2 legendaries are added.
Legendary birds will be all weaker than Snorlax, Lapras and even draconite for defense. They got poor defensive movesets : low damages quick attacks (fine for artico
Uno) and no type counter attacks like icebeam for waterpokemon. Zapdos will be very usefull for attack and articuno for defense and attack. That's it