I mean... yes that would be convenient, but I think it’s far too late to pull a Berlin and switch from several smaller termini to one massive one, since there’s no place to really put an entrance, and having an entrance to London’s central station be in the style of a tube station would be kinda underwhelming and people would have a hard time finding it.
Possibly the only time that there may have been an opportunity for this, in terms of surface land availability in recent years, would have been to use Battersea Power Station.
It could have been quite spectacular, but not in the least bit central, and hamstrung by dismal connections and nowhere near Crossrail.