My opinion is that Google/Mozila is pushing HTML5 instead of plugins not because that's what's best for the web, but because exactly this-- it decreases the potential for competition.
A big counter-example is Opera Software switching away from their own engine to WebKit/Blink.
Opera has a paid staff of full-time engineers, which makes rewriting a browser on top of a new engine a lot more feasible.
You're right in that if Camino had the developer time it had in its heyday it might have made the jump and survived, but the end of Gecko embedding is what made it impossible for Camino to continue to be maintained indefinitely by one or two people.
What if you only needed to implement html1.0 and NPAPI?