Sometimes it's healthy when spaces heavily fragment due to hypercompetiton. It allows products with strong technical and theoretical groundwork to show up later and render the entire space effectively obsolete.
1. Market: If the pie doesn't grow, you may be right. If, however, these become new platforms, with the majority of use cases and user value occurring in the future, then there's certainly room for many existing players to succeed, and for brand new players to. Most people don't even take notes. What if knowledge management looks different in 10 years?
2. Consolidation: Notetaking is pretty singleplayer and subjective right now. Until there are "network effects", reasons why people would want to purposefully join the same network, a lot of it will simply come down to taste. Note: network effects does not simply mean real-time collaboration.
Another use case that goes beyond individual notetaking is collaborative knowledge management for organizations. So far Roam's collaborative features have been lacking to say the least.
So basically a Github vs Gitlab scenario. Both are extremely valuable companies. One is more for individuals, the other for enterprises. Knowledge management can go broad and deep, and we're only processing and collecting more information. Never underestimate a growing problem/market.
It's possible that sometimes we have to let early ideas fully succeed to see how the market actually performs. Else one can end up in an arms-race situation that ultimately only leads to a tiny user base for each company.