This is the same logic that got XC into trouble capacity wise 11 years ago...
It is partially true but it will always remain the case that if X number of people like to use the 12:00 from Anytown to Somewheretown in favour of the 14:00, making it half the size and providing a 13:00 departure as well won't result in half the people on the 12:00 and half on the 13:00.
Personally I think the best use of the 222's is on XC. XC passengers already have low standards and low expectaitons as a result of more than a decade on noisey Voyager DMU's anyway, so it would be seen purely as a capacity increase and not a downgrade. They are not compatible with 220's or 221's but:
a) Is that really insurmountable?
b) HST's are not compatible with 221's either and XC run those
Personally I don't see this supposed massive surplus of IC DMU's appearing - just because wires are up doesn't mean they'll stop using DMU's. After all, there are numerous services today operated by DMU's under 100% electrified routes. I was on one today, infact.
Although it follows a simmilar logic to XC, there is the possibility of more 222's than what are required to run an hourly (short form after a split) service to Cornwall and back, which was never the case with XC. Even if there is a delay of 5 years before all the 222's are avaliable to run the service at full length all the way through as they are still required on the MML for a bit there is still the possibility of more stock.
Also although I was pointing out that even by using the 5 coach trains there was enough capacity on an hourly frequnecy to cover the number of seats which the current 2 hour frequancy provide I was assuming that some services would be covered by the 7 coach sets and some of the very busy services could even be run as full length (5+5 or 7+4) trains, which would provide for greater capacity than there is at present.
It maybe possible to make the 22x's compatable, but given that XC are could be getting IC EMU's and/or ICWC may be getting rid of their 221's there is proberbly not much need to run the 222's with their 220/221 counterparts.
There was no need to make the 22x's compatable with the IC125's for 2 reasons, firstly the IC125's are only used because passenger numbers have grown (and possibly some bean counter thinking that there was money to be saved by buying a few less than where needed) as it was likey that the orriginal plan was that the 22x's would be enough to do away with the IC125's. Secondly, there is no need for the IC125's to run with any other trains as they are a full length service, whilst the 22x's are short enough that they can run paired up (e.g. 4+4, 4+5, 5+5 and 4+7).
Yes there are diesel services which run 100% under the wires, however a lot of this is due to the fact that there is insufficiant EMU's to run all the services required, yet there isn't a large enough order for the purchase of more EMU's to make it possible. Add to that the 22x's are on a lease which means that they have to be paid for even if they are not used and it makes it cheaper not to run the cheaper to buy and run EMU's, so no one is looking to buy IC EMU's until this lease expires.
Also there is the fact that the number of totaly wired up potential routes in the UK is still fairly limited and so if a TOC is going to have any sare trains they will make sure that it is one that they can use on any service (i.e. a diesel). As time goes by and more routes become fully wired up it becomes more likely that EMU's become the norm for IC services.