I don't entirely agree. Historically, a 'standard' lens was one that was close to the diagonal of the negative, and with 35mm the 'long standard' became the norm because it's easier to make a reasonably fast (f/3.5) 50mm lens that covers the frame well than to make a 43mm. If you want faster still (f/2, then f/1.5) with full-frame coverage, 50mm is MUCH easier. Anything longer, on the other hand, soon gets bulky if it is at all fast. In other words, it was cheap'n'easy.
Degrees of enlargement also enter in to it, and I'd argue that a 35mm is at least as 'standard' as a 50mm, while 40mm is closer still. As for 'the most generally useful', I'd disagree completely; I find a fast 35 much more generally useful, and I am not alone in that. As I said above, my 'standard' lens is a 35/1.4.
Then again, my favourite 'standard' on the Nikon F was the 58/1.4 -- like many ultra-fast lenses for reflexes, even longer than 50mm for similar reasons to why 50mm (or 2 inch) lenses were normally supplied instead of 43mm, 40mm or 35mm.
Cheers,
Roger