Sure, but if the only thing to weigh is "will we lose users if we don't implement this feature?" and the only reason given why it's different this time is the a

Sure, but if the only thing to weigh is "will we lose users if we don't implement this feature?" and the only reason given why it's different this time is the assertion that this time it's different, I think you can understand the sentiment. Besides, I think the argument is more "Either Mozilla doesn't compromise where it's important not to, or they might as well go away", which is a true statement. Mozilla does need to continue to exist, and compromise is fundamental to the web (it's right there in their manifesto), but this was something that should have been fought tooth and nail. The protest of this mostly involved sternly written emails to W3C lists, when it should have been simply not implementing it and holding that ground until it was clear no alternatives were viable and imminent harm was actually demonstrated.