I agree it's not a large-scale bubble, but I think it will still qualify as a small-scale one -- a "bubble in a teapot", if I may mix metaphors -- if a lot of a

I agree it's not a large-scale bubble, but I think it will still qualify as a small-scale one -- a "bubble in a teapot", if I may mix metaphors -- if a lot of angels invest in low-quality deals and run themselves out of money. There's a key difference, though, between this situation and one conducive to bubble formation: the successes of the deals aren't correlated. The success of one startup doesn't make the next more likely to succeed. It may cause the next to attract more investor interest, but once it gets going it still has to succeed or fail as a business. Thus the valuations are still ultimately grounded in reality. Also, it's not the case that a startup is passed from owner to owner the way that, say, real estate is. So I prefer the alternative explanation that we as a society are simply getting better at nurturing startups. (I.e., I'm agreeing with you.) That certainly is sustainable.