Replying here as off-topic in original thread.
The suggestions for HST replacements in general are based on the fact that the unions (rightly) are rather unhappy about the safety of HSTs, as shown by the Carmont crash in particular. The HST replacement post-electrification is many years away, so suggestions have been made to replace them earlier, and the 222s are available soon, so have been floated as an option.
However, Alex Hynes stated on the railnatter podcast that the HSTs are currently being modified to improve their safety, and that they are planning their replacement for 2030/2035ish, so it sounds like the HSTs aren't, in fact, being replaced (source from 1:18:53:
https://www.youtube.com/live/00sj14IF144?feature=share&t=4733 )