The reason you can't run as many other trains on WCML and other lines is because high-speed non-stop trains take so much capacity. Once you remove them, you can run many more local/regional trains with more stops and higher frequency.

The whole way HS2 is designed is to maximally reduce the amount of fast trains going north south on the existing network. Leading to a massive capacity upgrade on the existing lines. You can still run some express lines but likely much more lines that stop at more station, making it fast for you to go to next HS2 stop and from there to the further distance destination.

HS2 connection to Leeds was designed to help the ECML, the whole HS2 system was designed by experts to help with WCML and ECML.

Of course now that the former car brained fucking moron of a prime minister in his last attempt to safe himself canceled most of HS2 all those benefits are gone. And labor is to cowardly and ignorant to bring it back.