Heavy:
Taking away Resurrect as an ability would either mean letting her mid-game be full of relatively nothing except target prioritization, or it would mean balancing it out with a new ability and then we’re in the mess of adding new powers to characters again, and just… that’s a mess.
I can see how this would possibly lead to unfun/boring gameplay when the only thing you could possibly do as Mercy is heal and damage-boost.
Heavy:
Adding Resurrect to Valkyrie is also not a good idea. For one, by adding more power to her ultimate, you make it even harder to balance the rest of her kit. Valkyrie is already a strong ultimate, adding more power to it via this extremely precious utility is just going to make it unbalanced, or make it extremely difficult in tweaking other parts of her kit – you can’t justify buffing other parts of a hero’s kit if they need it when that hero has such an amazing ultimate.
I’ve seen Eevee’s post about how Valkyrie can be tuned down a bit and make it more balanced. I’ll put a link to Eevee’s post here: Suggestions for Mercy from someone with too much time on her 📝
Honestly, Eevee has great suggestions on how to balance Valk, such as reducing the duration from 15 secs to 12 secs.
Heavy:
Furthermore, you run into a problem where no one would use Valkyrie in situations outside of ressing someone. I mean, why would you? Why pop Valkyrie if no one is dead? You’d be wasting that useful ability otherwise, so this limits when and where Mercy can cast Valkyrie. Right now, the whole design of the ultimate is to give Mercy independence – her greatest weaknesses are temporarily relieved and she isn’t so bound to just pocketing teammates. It’d be a bit contradictory to add a new power to this ultimate that restricts when she’s allowed to use it in the first place.
I don’t think players would just use Valk solely for the Rez ability. I think Valk is already a good ult for keeping teammates alive through chain healing, although Mercy’s healing has been/will be reduced from 60/s to 50/s soon, so currently, I can’t say how useful Valk will be in the future.