I don't think diesel is preferable, but it's also not a straightforward either/or. The construction of new electric trains also has an environmental impact, as does scrapping existing trains. 40%+ of electricity comes from gas power stations too.
But electric trains are being constructed for other lines. Perhaps some lines which don't need them as much as WCP does. And diesel trains are being constructed for other lines where perhaphs more chopping and changing could have occurred.
Voyagers could possibly be leased to another franchise if new trains were procured for WCP .Certainly nobody would scrap them. But get them out from full operation under the wires. Like I say 170s are being replaced wirh new trains and moved where leases are up. And they are the same age. But they'll be moved not scrapped.
Whatever the lease dates are for 221s and 390s will presumably have some effect on where and how long they stay. I suppose in theory a TOC could take them for the right price and run them wherever if the price was right.
"What's that, you'd give us £x millions to run 221s on the Whitby line? Oh... Go on then!"

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Well I'm guessing the Voyager leases are in some doubt for either WCP or XC or both. As why arent they having their midlife overhaul like the Pendolinos are? They have been out just as long as the Pendos, longer on XC.
Also, voyagers could be put to good use on XC as far as Cambridge, if it was trimmed back from Stansted. Or XC to Cardiff or Nottingham. Or the Liverpool Norwich, Chiltern, routes into Wales . Potential work on TPE. The list goes on. They wouldn't need to be scrapped.
Besides that XC might want any spare sets from WCP to double their trains. Which is a separate subject altogether. But fact is they may do.