I've made quite a few journeys in the Derby - Leicester corridor since the beginning of this year; times have varied.
With regard to MML loadings, particularly north of Leicester, whilst I have always been able to find a seat easily southbound with one exception (on one which started from Derby at about 07.30 and was formed of an ex Grand Central HST), all of the trains on which I've travelled have been well loaded and have usually been formed of 7-car 222s; however, on at least two occasions I've been waiting for the 16.05 northbound service at Leicester on a Friday afternoon when there has been an announcement on the station's PA warning passengers waiting for the 16.02 southbound service (I think a 5-car 222) that there were already people standing on the train. On each occasion that I have travelled on the aforementioned 16.05 northbound service (usually a 7-car 222), there have been significant numbers joining at Leicester and standing north of the city and as far as Derby has been observed on a number of journeys.
The majority of journeys I've made have been on Fridays, and I appreciate that this is often a busier day than other weekdays; however, a couple of months ago some friends made enquiries about travelling from Derby to London on a Saturday morning, and the first train on which they were told there were free seats was after 12.00!
So based on my experiences and observations over the last 7 months, it is sensible to procure a fleet with more seats that are presently provided.
Regarding future abstraction of traffic by HS2, I believe assumptions made regarding transfers from MML services are over optimistic with regard to Derby and Nottingham; Toton is a difficult place to get to by road, tram from Nottingham will be slow, and if a "shuttle" is used for a transfer between Derby/Nottingham and the HS2 hub, overall journey time savings to/from London will be small. Toton will be a large station, and 10 minutes for interchange is reasonable, and to this must be added, say, 13-15 minutes for the shuttle journey; this means that - on a fast Sheffield - passengers joining at Derby would already be approaching Wigston before they'd left Toton, and from there to St Pancras isn't much longer than the projected journey time between Toton and Euston.
So whilst the opening of HS2 will be after the Abellio franchise period will end, the bi-modes will only be at about half-life if it opens in the late 2030s, and I believe that a fear that too many seats might be provided now for future demand is unfounded.