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