Alan Gunn comments:

It's not possible, or even desirable, to stop English from changing in this way, but why should we encourage it, or think it always a good thing? I'd just add that changes like this not only make understanding more difficult during the transition, they end up making older writings hard for modern readers to understand. And some of the changes are downright ugly: to me, at least, an ordinary English word like "happen" sounds better than "transpire." (And I suspect the people who like words like "transpire" of trying to talk down to people who use normal English words. Lots of them seem to have gone to expensive schools, and to talk about their schooling at length.)

As it happens, I don't like "transpire" to mean "happen" for reasons similar to Alan's: "Happen" sounds simpler and less Latinate. But claims about how some new usage is supposedly a "change" -- especially, by implication a recent change (since all usages are novel if you go back far enough) -- always make me want to go check, preferably in the Oxford English Dictionary. And here's what I see in the OED:

b. Misused for: To occur, happen, take place.
Evidently arising from misunderstanding such a sentence as 'What had transpired during his absence he did not know'.

1775 A. ADAMS Let. 31 July in J. & A. Adams Familiar Lett. Revolution (1876) 91 There is nothing new transpired since I wrote you last. 1804 Age of Inquiry (Hartford, Conn.) 46 When..the reformation transpired in England..almost the whole nation rejoiced. 1810 F. DUDLEY Amoroso I. 14 Could short-sighted mortality..foresee events that are about to transpire. 1828 WEBSTER, Transpire..3. To happen or come to pass. 1841 W. L. GARRISON in Life (1889) III. 16 An event..which we believe transpired eighteen hundred years ago. 1848 DICKENS Dombey xxxii, Few changes{em}hardly any{em}have transpired among his ship's company. 1858 HAWTHORNE Fr. & It. Note-bks. I. 225 Accurate information on whatever subject transpired. 1883 L. OLIPHANT Altiora Peto I. 277 His account of what transpired was so utterly unlike what I expected.

A few thoughts: First, the OED does say that the word is "Misused for ... happen," a rare bit of what looks like prescriptivism. Second, the "change" seems to have happened at least two centuries ago; the OED doesn't tell us how common a usage was, so maybe it's become much more common recently, but there certainly are plenty of attributions -- and not from obscure sources -- going back to the 1800s.

Third, the misusers include, among others, Dickens and Hawthorne. So the word has commonly been used in a particular way. It has been used this way for a long time. And it has been used this way by some of the leading English-language writers. How then can we report this as a "misuse" as opposed to just a use (or perhaps a use that originated from a misconception, though that hardly makes the use a misuse today)?