I'm a freelancer and deal with many small website owners. They have no idea about php versions, deprecated functions, etc. They want to run their business (online), and keep using code that satisfies their needs.
I'm already seeing issues with the move to 5.4 - having to check client code (most of which isn't stuff I wrote) and fix up deprecated functions. In some cases, the original software is no longer being updated or supported - so even though it works, the business owner is stuck with the prospect of paying to have it fixed, or installing a new solution. Or finding a provider that will support 5.3 out in the future.
The php team is really blowing this one. I have clients running perl code from 15 years ago, still hums along, never had a problem like this. With php, my clients will have to pay $$ to upgrade for NO additional functionality - just because some propeller heads live in a fantasy world where everyone codes 24 hours a day, and rewrites their entire codebase every year or so.
What will happen is that hosting companies will not be able to upgrade to the first PHP version that kills the MySQL extension, or else customers applications will break and the customers will move to a new hosting company that offers them a PHP version with a working MySQL extension.
Basically hosting companies will be in control of what PHP versions customers will use, not the PHP core developers that decided to kill the MySQL extension.