Streets of London has the esssential quality for becoming a folk song - it survives in the wild. People sing it without any idea of where they heard it, including those who've never heard of Ralph McTell.
Once again, I think the manner of presenting it often gets in the way. Sing it pretty, with a pretty guitar accompaniment, and it comes over as a charity worker preaching, and putting himself too much at the centre. I'd read it more as a dosser talking, about other dossers.
Probably works better from a drunk outside a pub, who can't remember all the words. Like Carrickfergus in that respect.
I'm all for folkies leaving it alone. It'll survive and mature.