You can run two “off healers” if your team comp isn’t built around taking a bunch of damage and needing heals though an extended fight. You can still run a classic dive comp successfully, for example, with Lucio and Zen as long as your team is coordinating dive targets. When a fight is over quickly, heals aren’t nearly as necessary as utility.
Mercy’s base kit is mostly the same but Valkyrie gives her many more options for keeping her team alive. Mass resurrect didn’t do anything at all unless her team was already dead. It’s the only ult that literally couldn’t be used at will. I’d rather keep my team alive in the first place.
I can’t say enough about how much I love Valkyrie as an ability. If you want to see that it does so much more than you’re giving it credit for, here’s a thread I made a little over a year ago showcasing some of what it can do.