If it was something like a database engine, an obscure language, some network utility, a server, etc, I might have agreed. That is, if it was something esoteric, that needs high skills, and only some very knowledgable of it's internals could adopt it.
But this is jQuery we're talking about. Not rocket science. Lots of guys have the skills and time to fix these problems.
And wouldn't the same exact reasonings apply to the jQuery team? They can also could think "of a million things they'd rather do than fix IE6-7 bugs".
Still, they said the will release 1.10, and support old browsers for as long as needed. If they hadn't released 2.0 that would also be the case. They would support old browsers for as long as needed. The only difference is now they are doing it with two codebases.
That said, your company might not want you "spending company time fixing a bug in a plugin", but for popular plugins there are companies with the money and the resources to have their stuff fix them if the need arises.