Nah, in October 2010 the first G1 owners' two-year contracts will start ending and they'll get offers for cheap or free upgrades to newer phones. Within a year or two this will happen to most people who bought a first- or second-generation Android device.

At least in the US, the carriers' subscription and hardware subsidy model will guarantee steady turnover in smartphones.

Or just don't support it. Most phones running 1.5 are underpowered, and if users haven't upgraded they probably don't care about being on the cutting edge and aren't buying many apps. I'd expect that the effort isn't worth it unless you have huge sales volume.

Nope.

There is nothing preventing people from abandoning Android 1.5. Contracts run up and people are thrown fancy new phones for free. Nobody has built mission-critical business apps around Android 1.5, as some big businesses have ActiveX/IE6.

Android 1.5 will simply be discarded. It's already closer to that than IE6 is!