This was done before, and was a disaster so they changed it back.
Splitting is the solution to this.
The journeys concerned are longer distance fares, as the shorter ones are 2V anyway, so it will be easy to get around the restrictions, though how easy/cheap that will be, depends on the flow.
From places like York it will be fine, as Northern and East Coast have a relatively sensible policy (unlike XC) when it comes to pricing before 0930, and by the time you get to the Midlands it's not too bad with EMT (worse than EC/Northern but better than XC!) but people who start their journey in solid XC territory like Birmingham New Street may have to pay a bit more.
For example, a York to Plymouth Off Peak return is £142.10, so if XC want people to pay £286.00 for an Anytime the clerk should politely (it's not their fault that XC are disgrace, and XC don't have any ticket clerks anyway) be told that they will not be getting their hands on your hard-earned extra cash, and instead ask for an Anytime return to a station nearby, and then an Off Peak return would start after 0930. So, if I wanted to get the 0824 from York, I'd get an Anytime return to Sheffield, an Off Peak return from Sheffield to Derby (yes, that would be departing Sheffield before 0930, but that's fine as EMT price that ticket and it's valid from 0900!) and then the Off Peak return from Derby to Plymouth is valid as by now it's 0953! The total price of these 3 tickets is £149.20. Ask for combinations like this at York and the staff won't be at all surprised at the request, they expect to be asked for combinations and are quite used to it!
So the customer only pays £7.10 extra, not the £143.90 extra that XC wants. And the best bit? XC will get even less of the money, as the combination of fares will give considerably more money to other operators for the first two tickets! In many cases XC will be reducing their own revenue.
Certain organisation and individuals (who are clueless but falsely purport to represent customers interests) want peak restrictions to be "unified" and "simplified" across the entire network, e.g. all after 0900 or 0930, if that happened then splitting would need to be done for every long distance journey before that time!
At present, you do have to do the splitting trick for medium distance journeys, but not really long ones. Under XCs new pricing, you will have to do this even more!
Note that I do not know if York <> Plymouth is actually going to be affected, it's just an example.