Even though there is less competition, the fact is, that it isnt balanced, if they were all picked approximately equally among those choices, fine, but that isnt the case at all, those 2 in the past and to a lesser extent (thank god) now still get picked waaaay more often than the rest and both are extremely powerful.
And Mercy and D.Va are good on their own and synergize with any composition. They may do better in some over others but the fact is they are not like some heroes you have to play around, they can do well in any situation, but they do that too well.
I agree, but while I dont think that Mercy needs nerfs at this point, in fact if you want an extensive look into my thoughts on here you can see this post I made;
https://us.forums.blizzard.com/en/overwatch/t/yet-another-mercy-thread/355676