they have enough 165s, just not enough 168s, they only need the 165s for aylesbury via amersham and aylesbury via high wycombe (although the latter should just be the risborough shuttle in my opinion but i wont talk about that), before 2020 it was 168s doing all mainline services bar the aylesbury stoppers, nowadays the 168s only really do the birminghams, with 165s doing the oxfords, however it does make sense as 165s have more capacity, which is good for bicester village as that can get very busy. Yes, the 172s didnt help much but nowadays they have also lost a 4 car 168 and 2 165s, which which is why getting 170s from northern makes sense