I can see both sides of the argument here — from the technical point of view, it is certainly unappealing to maintain such kludges, and from the business point of view, "it's been working fine so whatever changed on the other end is broken, we won't spend money fixing it"…
Do you have any thoughts on how this quagmire can be avoided on the web? I can imagine solutions similar to Ghostery's stub scripts, but putting that in a browser, let alone many browsers, sounds like a large legal problem.
Aside from something like forced namespacing, I don't, really. I suppose being able to pin the JS version in the browser might help, similar to quirks mode of old, but that leads to another version of the BC issue.
But ultimately this is a question of the lesser of two evils, and I do sort of think that the real solution here is that site owners own their sites and their decisions as to what libraries to use, and thus own their own downtime. Browser vendors can't reasonably try to take that responsibility on themselves in an uncurated ecosystem, especially in cases like MooTools where the library vendor did something long known to be questionable.
That may be idealistic, but I think it's the only path that really works in the long run. Unfortunately, in browser market share is king so I doubt that attitude will be adopted. But I definitely wouldn't buy any argument that it's for the user's benefit--it's all about not wanting to get blame splashed back.
Do you have any thoughts on how this quagmire can be avoided on the web? I can imagine solutions similar to Ghostery's stub scripts, but putting that in a browser, let alone many browsers, sounds like a large legal problem.