Most commericial web sites already deal with this issue - they have new/beta features that are initially only visible or available to a subset of users. Google Apps lets you choose if you want the new or old version... it's not impossible to let users stay with a static implementation, but it's just generally not worth the trouble.

Conversely, native apps suffer from the problem that once you deploy a package out the store, you have to support it (unupgraded) for the foreseeable future. That creates its own bucket of issues.

Agreed, both are trade offs. One is not "clearly a winner", it depends on your goals.

Oh absolutely; but when did I describe it as straightforward? If it was such an obvious slam dunk win it would have happened faster and Microsoft would not have been caught with their pants down. The gist of my point is that over time, you have a cross-platform, no-install environment that clearly has a ton of disadvantages, but it cleans the clock of superior established platforms simply due to a little less user friction.

This argument is often advanced by people against automatic updates, but I would suggest that its such a small minority of situations and people that its not worth considering unless you operate in a very specific niche.

You version. Move the old version to url/oldversionnumber and tell that client that they can keep using the same old same old at that url.