The problem is keeping pace with the upstream: it's not that it's impossible but that it's very expensive — when Google forked WebKit they dedicated a large and very talented (i.e. expensive) engineering team to the project. The same could be done again but you're looking at companies like Microsoft, not startups.
The resources needed to maintain a Gecko-based or Blink-based browser will depend on the amount of customisation. Vivaldi/Opera/Brave are doing fine, but they make relatively shallow changes over Chromium.
I just discovered Goanna on Wikipedia, a fork of the Gecko engine, presumably with relatively thin resources. Don't know how well it compares to mainstream engines though. [0]
I suppose the short version is that the workload is a function of the goals.
That’s the point: if you’re not customizing Blink, you’re not changing the huge influence which Google has over de facto web standards. If you want to make more than simple customizations you need a significant commitment just to keep pace with the upstream – Microsoft can afford that, Samsung can, etc. but it’s not clear that Brave or Opera can.
