What do you mean "lack of accessibility of Gecko"? Did you mean "lack of embedability"?

Gecko's embedability sucks ... on desktop. But that's not inevitable, it's just a matter of priorities and resources. On Android it's getting a lot better and something like GeckoView on Android could be made to work on desktop too ... if there's demand and perceived benefit.

(Mozilla's in the middle of upgrading Gecko's multiprocess support for fine-grained "site isolation" so now might not be the ideal time to stabilize embedding APIs.)