Servo was never meant to be an alternative browser engine that would replace Gecko.
It was an R&D project and, if you look at the bits that made their way in to Firefox, a fairly successful one. Stylo, WebRender, various parsers etc...
I know better than to dispute Alon Zakai on Mozilla internals, but it remains true that Mozilla's external messaging unwaveringly insisted that Servo was not going to replace Gecko. Even in the later years it emphasized the strategy of gradual replacement of select components.
Interesting. I may be less aware of the external messaging than you. Do you have some examples of that perhaps?
I'm not doubting you, I'm just curious what was written. I'd expect stuff like "Servo is an experimental browser engine," or, "We don't currently have plans to replace Gecko with Servo" (which was obviously true at all times in history), but I'd be surprised to see something like "Servo will never replace Gecko." But maybe I just didn't see that messaging - could be!
It's also worth mentioning that "replacing Gecko" may be the wrong way to look at it. More reasonable is for Gecko and Servo to eventually come into alignment - somehow. That's what happened in practice, in a way that has a lot more of Gecko than Servo, but in theory it could have happened the other way, with a lot more Servo. Even in that situation, I wouldn't say Servo "replaced" Gecko - it's more complicated than that.
For Mozilla, Servo was an experiment. Experiments can end up as full products, or parts of them, or not at all. I think it's clear one possible outcome of the experiment was a full browser (I was personally rooting for that all along). It didn't end up that way, but when people say a full browser was never the intention, I think that's not accurate history.
Not off hand (and Mozilla's official marketing rarely goes so low-level as to mention the browser engine itself by name), but (as someone who has been hanging out in Rust spaces since 2011) I recall the Servo devs always being very careful to disclaim any intentions to wholesale replace Gecko in Firefox (though I did always wonder if they were instructed to do so, as a way to keep Gecko contributors from rebelling).
Yeah, I also didn't see a culture of saying "we're going to replace Gecko." Not because there was any instruction to speak carefully AFAIK, but because that's not the goal. The goal was for Servo to succeed, hopefully both technically and as a product.
Product success could happen through convergence with Gecko (as actually happened, but again, the convergence could have been tilted the other way), or through Servo powering something separate from Gecko (for an example of that, in later years Servo was meant to power Mixed Reality projects, which is a story in itself; but other ideas include Servo as an embedded engine, or powering distinct tabs in Firefox and other obvious ideas).
In some of those options Servo could have been a full browser engine or even a full browser. I think there was always a hope for that. I'm sad it didn't happen, but it's still a big success in several important ways. Anyhow, I just hope the history doesn't get rewritten as "the goal was always exactly what happened in the end."
It was an R&D project and, if you look at the bits that made their way in to Firefox, a fairly successful one. Stylo, WebRender, various parsers etc...