FWIW, the use of singular they as a gender indeterminate pronoun is very well-established, in fact the revisionism is in the movement to cast the form as ungrammatical:
Look at the reflexive form in that article. "Each child feeds themself."
I don't think that singular "they" never be used, just that enforcing its usage everywhere by claiming that the opposite is "hostile" (as this PR seems to lead to) will lead to awkward constructions.
I don't advocate enforcing its usage, I just think it is more appropriate in these circumstances. The reflexive form isn't well established, but I don't think that particularly matters.
Languages are messy, and its difficult and futile to enforce prescriptive logic on them. To build upon your example, if you search for the form 'learn for themselves', you'll find that many people use the phrase with nouns which are nominally singular, such as 'this is something that each generation must learn for themselves'. Well, so be it.
http://en.wikipedia.org/wiki/Singular_they#Generic_they
This isn't really anything like 'theirstory', which is an artificial construction.