I agree, partly. The complexity problems stays and times when you even could think about starting a browser as a one-person-project are long gone. Even if you could get to the point of investing e.g. five years all the standards and possibly use-cases change so fast that it seems nearly impossible to "start" a new browser engine with some prospect on market share.

A friend of mine in web-dev also told me that most of commercial , "big" pages are heavily relying on web-services of amazon, google and cloudflare so you have to comply with their practice-standards as well if you want your browser to be able to display any of these major pages.

Maybe the big five now don't completely control the browsers but they surely control most of the hosting and software that fuels most data centers.