By the end of 2022 the 222s and XC 221s are off lease - these can pretty much fill in any shortage of trains an HST could fill, and are compliant with PRM and toiler emission legislation, which the HSTs aren't. Given the current situation, the chances of a chronic shortage of trains before 2022 seems pretty low, so scrapping them doesn't seem stupid.
Correct
There has been a dearth of diesel powered passenger trains for the last ten years which could have been ameliorated by the Diesel Trains Ltd Rosco that the Government were going to create, then didn't. So Porterbrook chose to invest millions in converting EMUs to DEMUs partly due to the then impending PRM issues. Several other solutions by the DfT could have resolved the imbalance of diesel traction by now - such as the TOC sponsored electrification of Crewe-Chester which would have allowed
390s to work through to Chester
TfW nee ATW to use EMUs on Crewe Chester shuttles.
Completion of OLE Didcot-Oxford would have released more Turbos westwards
Completion of OLE-Reading-Basingstoke would have released more Turbos westwards
etc etc
However the restocking of Franchises with Hitachi IETs is releasing the Avanti 221, all 222s and four 180s in the next two-three years. Also for consideration is the on/off debate about surplus 185s and the fate of the TfW 158s.