(I do imagine that there should at least be ample precedent out there for the "management of ~10% market share company decides to sacrifice strong point of its own product to pursue the strong point of the market leader instead" pattern. I can think of several failed examples, but no successful ones - are there any? If not, I believe that should count as evidence that this is a common bias in company judgement.)
a) Favoring extensions over browsing experience (rendering speed, jank, etc) is likely to improve Mozilla's market share.
b) Market-relevant extensions are not going to be ported to WebExtensions.
Speculation and personal preference is not persuasive.
...and regarding a), I don't think that's all there is - FOSS projects at this point have a quite long track record of changes that were championed by the marketing crowd over the skepticism and outright opposition of the "fanatic techies" of the kind found here, such as KDE's Plasma Mobile and Mozilla's own phone OS, which turned out dead in the water exactly as the "unrepresentative" crowd expected.
[1] https://addons.mozilla.org/en-us/firefox/addon/downthemall/
[2] https://mail.mozilla.org/pipermail/dev-addons/2016-December/...
This is the same as saying, "The judgment of the company is better than the judgment of their customers who say they will stop using their product if the company follows through on their plans."
That's the bet-the-company question, isn't it? Only history will tell, but I think the judgement of Mozilla in this matter is better than that of the commenters on this website, who have less data, represent a minority of users, and are prone to complaining.