Such fragments don’t need to be particularly human-readable, only machine-readable. Given that, greater flexibility is generally a virtue: different matching strategies will work better in different contexts, and different tools can benefit from it. Consider the increased scope of the annotations feature—it’s designed for things like more robust referencing, bookmarking with annotations and other things like that.
There then remains the question of cross-compatibility: do all relevant tools implement the same techniques? That is a legitimate concern, but it’s well-enough specced that it shouldn’t be a problem.
Also as noted, various tools out there already use this format. The Chrome team ignoring that and using their own, functionally inferior format is hostile.
it adds a huge amount of unnecessary complexity for such a simple feature
There is a reason nobody implemented the W3C proposal, If I have the choice between a simple working solution that anyone can describe in a single sentence specification and a hundred page specification with tons of unneeded features that nobody is going to implement ever, I choose the former. I wonder how much hate Mozilla would've gotten for the same feature, probably none.
No, Mozilla would've gotten plenty of hate for potentially breaking sites, and adding features without discussing things with other vendors and Internet bodies. The thing is, we all know that Mozilla can't get away with that, but Google can, given Blink's market dominance.
But various tools have implemented the W3C proposal; just not browsers yet, probably mostly because getting the UI right is tricky, and the risk of breaking things higher. The problem is not in the particular syntax, but the feature as a whole.
There then remains the question of cross-compatibility: do all relevant tools implement the same techniques? That is a legitimate concern, but it’s well-enough specced that it shouldn’t be a problem.
Also as noted, various tools out there already use this format. The Chrome team ignoring that and using their own, functionally inferior format is hostile.