This has been covered multiple times in this thread and others.
Remember the EMR fleet decision was made in an environment of competitive tender where DfT's main interest was maximising premium payments. A bidder that proposed running full-length trains all-day would have lost the bid. EMR is the shortest of all intercity routes, serving only medium-sized cities with no particular tourism merits, and just doesn't have the all-day off-peak demand like its East Coast or West Coast neighbours.
It's also easy to forget that EMR demand and capacity forecasts were made in an era when HS2 was expected to whisk away the majority of East Midlands demand by 2033. The Leicester demand remaining with EMR would never have justified running full-length trains all day.
These are all fair points.
However, sticking with the 222s for longer, and then ordering electric only units to be used after electrification (and HS2) came online; or alternatively, ordering 9-car "standard 80x bimode" (or standard 80x bimode with extra diesel gensets to increase off-wire power) for EMR that in a GBR world could have been used for GWR or LNER (as needed) or put towards XC services as part of a move to integrate XC more into other IC services.
Both options would have some merit, but I do understand why they were not selected.
What I really don't understand is why new non-standard, high power bimodes were ordered for a line that, at the time, was planned for full electrification, while there wasn't anything inherently wrong with the existing trains, other than that there weren't enough of them.