To be fair, even if let's say this new demand for extra services doesn't fully materialize, there is still a demand for 2+2 stock on Portsmouth and Poole services, because passengers on SWR have been going on and on about this for what seems like forever now. The 450s are great trains but there will always be an issue for as long as they are used on these longer distance trains. So something still has to be done.
The 442s weren't the most ancient things knocking around when Southern got rid of them, and they seemed like quite a reasonable option at the time. Granted perhaps it wasn't prepared for as much as it should have been, i.e. issues which were uncovered should have been identified earlier on, but as far as SWR were concerned the only dodgy thing was the traction motors, and the success of the 455 retractioning probably led them to believe that it could've been done and dusted in a couple of years, and the assumption probably was that they could carry on working quite happily until the end of the franchise and the next TOC could then deal with replacing them. So really, whilst mistakes were made, it's less of a 'terrible idea in the first place' and more of just a very unfortunate sequence of events.
The 442s were lovely but they should just be laid to rest now. I'd love to see one preserved, but not sure if anyone would take them in their current state. I liked them on the Brighton fasts as it felt like the BML was for the first time getting proper intercity units, even if they weren't as grand as before.
As for the 458s, I think this was the right choice. They aren't totally foreign to working mainline services, Reading services are perhaps longer distance but not high speed, but they do have some experience on Alton services, and possibly others but I'm not sure. Someone has obviously worked out that refurbishing these still works out cheaper than carrying on at this stage with the 442 project, so this move will probably save a bit of money still, and don't forget that they are only a few years older than the 450s, so hypothetically if kept in good nick they won't need replacing until the 450s do.