And it's not as if there's anything wrong with the 332s either, which seemed perfectly suited for the HEx route. An unscientific analysis perhaps, but I don't recall a single thread beforehand talking about the possibility of 387s replacing them.
Anecdotally, the reliability has dipped massively over the past 2-3 years. 332s used to be virtually bullet-proof, major failures in traffic simply didn’t happen. More recently there’s been at least 2 major failures on the mainline in the past few months, short-forms seem to be becoming more common, there was the “recall” issue last spring with the bogies, and they didn’t want to come out and play in the recent snow. The fleet is worked very hard, and it’s starting to show.
They were never going to fit ETCS to them - the electronics architecture just won’t support it. GW-ATP is becoming life expired, the system is nearly 30 years old. Even if the GWR/HX deal hadn’t been proposed new stock would have been needed in the coming years - AGA 360s may have been an option; with sufficient additional units for services West of the Airport.
Going forward there are two stumbling blocks.
1) Ownership. The trains are directly owned by Heathrow Airport. ROSCOs already have a glut of EMUs coming off lease, many of which are considerably more capable and modern than 332s.
2) Technology. The trains are not fitted with TPWS or GSM-R. Both of which would be required for the units to be productive elsewhere. The interior-fit is very “luxury-Airport-express”, again very few places they would be useful. They’re also not particularly quick off the mark by modern EMU standards.