Considering an investment firm bought them all (220s & 221s) in 2017, they would be expecting to get more than a few years of income from them.
https://www.railforums.co.uk/threads/voyager-trains-sold-to-beacon-rail-leasing.151333/
Class 222s are owned by Eversholt
Current franchise is due to end in October 2020, but with apparently no invitation to tender having been issued yet, I expect the franchise to be extended. (seems to be 1 year between tender & franchise award, & 1 year extensions), so that pushes it out to end of 2021.
It took 3-4 years to get the Voyagers built & running, so you'd be looking at 2025 onwards for new trains for XC.
Which runs into the issue of what could you order in 2022? A diesel only train - no.
A bi-mode - to use what wires? What extra wires on XC routes will exist by 2025? by 2030?
Run the 22x's until 2040-3. That means ordering new trains in 2038, by which time there has been another 20years of electrification, & 20years of technological developments.