Once that is true, you want as many people to collaborate as possible and the browser is the perfect delivery model. Plus you ensure a consistent version across all clients.
Today the onus is on people who still believe in that idea to express why they think Mighty failed (without falling into the well-worn "year of the desktop linux" trap).
Personally I don't care about the idea either way, I'm just looking at is where the market is going (e.g., how the market share of various software packages are trending). Ideas that sound good fail all the time in the market.
And so far, outside of Figma, the idea you're expressing looks to me like a failure? And personally, I've started to look at other things different about Figma that might have accounted for its success.
And liveblocks.io is here to make that happen at scale.