Obviously it's a non-trivial fix when they engineered their system on the assumption that nobody should ever leave the iPlatform.
It might be a "stretch", but you could say designing your system in such a way that there's no clean way to leave is intentionally making it hard to leave. Or would that make you a lunatic?
Seriously though. Almost every ex-iPhone user I meet thinks SMS is broken on Android, because other iPhone-users (like wifes) no longer is able to message them. And I'm pretty sure Apple is 100% OK with that.
It's not a big problem to deal with. There is no motivation to fix it. It's not that Apple is doing anything purposefully. It's that they aren't, and won't, do anything to help since they know that OP (and that ilk) are not their customers. If OP didn't want to "lose" all his friends/network, he would have just got a 5s and shut up.
There's plenty of incentive for them to fix this. OP is far from the only one calling Apple customer support or going to a Genius bar over this issue. Those costs add up, unless they're also considering the incentives on the other side of the ledger.