I don't get what you mean by web standards, as chrome won't drop support for snow leopard, as there are is still a market base whom choose or use snow leopard.
As for the developers, I think more users will have to upgrade to ML, since newer applications might rely on newer API's, hence people will adopt the newer OS's.
Although some old applications might not work with ML/L, and they might choose to stay on SL. I think this question is really fragmented as its actually quite hard to distinguish what the adoption rate will be in the next few months and years. It's very user preference, you still see and hear people using Leopard, Jaguar etc. Sure, there will not be a lot of people using 10.3,4,5 etc, but if they are happy with the OS they have, then why do they have to upgrade?
But you could argue, that they should, or else it's just limiting how they are evolving. The technology would move without you, and it's something people argue everyday, you will never be in the loop of new technology for much time, 6 months in the technology world is massive, you can't think what will happen in 1 or 2 years down the road.
Web standards is tricky to argue, Google Chrome is a brilliant web browser that has always pushed major adaptations to HTML 5 and CSS 3, but Safari updates will stop for older OS's soon, but consumers can just download a thir party browser. Google Chrome, I think it will support Snow Leopard for a very long time, and I'm sure people won't really miss HTML 5 as its mostly optional right now and it's not finished yet, most browsers already have some incorporation already, but it's not a huge game changer for average consumers (yet).
TLDR; users will be users, they will upgrade if they want to, if not they will live in their own bubble, and keep using what they love. It's going to take a bigger evolution then lion and mountain lion to make a jump forward for an average consumer.