Not that there's much evidence for this with the Hitachi class 800/801 order: On the East Coast, there will be thirteen 9-car Bi-mode sets compared to fifteen 2+9 HST formations, as well as an additional twenty two 5-car class 800/801 sets: That's 93 carriages more than makes up the HST fleet at present.
30 x 9-car mark 4 sets will be replaced by 30 x 9-car class 801s on a like for like basis, but with 99 more standard class seats.
On Great Western, 21 x 9-car class 801s and 36 x 5-car class 801s, plus 7 x 9-car and 22 x 5-car higher rated AT300s, totalling 542 carriages, will replace fifty three HSTs in 2+8 formations and 5 x 5-car class 180s (plus an unspecified smattering of Turbos on the likes of Cotswold services): A total of 449 carriages. Or in other words, every Great Western HST could be replaced by either a 9-car class 801 or a pair of 5-car class 800s as the diagrams stand at present, and still leave three 5-car units spare if you also replace the 180s on a like for like basis.
I have my own doubts about the Hitachi AT300 series of trains, mostly concerning the proposed passenger environment, in terms of decreased luggage space compared to the trains they replace, seat to window arrangement and the comfort of the seats themselves: The latter two points being harder to qualify without having boarded one of these trains myself. I am not however dismayed by the number of carriages that are being built.