I've seen a startup struggle with so much waste from a bunch of senior engineers massively overengineering an app to the point of developing something akin to t

I've seen a startup struggle with so much waste from a bunch of senior engineers massively overengineering an app to the point of developing something akin to their own front end framework... before being anywhere in sight of profitability or even PMF. The pain lasted for years after, even when a successful business model started to come into view. With the exact outcome that you describe. Senior v. Junior is not the problem here. Developing according to your stage and future is. Build things that can be thrown away next week when your product shifts. (ie. low coupling, simple, avoid abstraction adventures). _Maybe_ "senior" devs are more likely to understand that. I'd argue it's rather devs with any multi-year experience at an early stage startup, regardless of skill level.