In fact this doesn't solve the problem of investors who wait and see. If they wait a week, and things look good, they can come back and say "we have this legally binding email that says we're in". Startup comes back and says "well we didn't get your response so as per PG's handshake protocol you're not in." To which their lawyer replies "what is this handshake protocol you speak of?"
Seriously, though, isn't this whole thing giving everyone flashback of interview questions involving philosophers sending stone tablets to each other trying to verify whether the other one got it or not?