Southern did have forty-odd class 455s, which were their metro fleet operating most suburban services – but then a coincidence of timing meant that they were hitting retirement age at the same point that Southern were reducing provision for a post-Covid world with fewer commuters, and so they were able to stretch the 377 fleet to cover the withdrawal of the 455s without needing to replace them. As they have needed to bring back a bit more capacity in the years since, it has made more sense to transfer in various flavours of Electrostars rather than getting a new microfleet.
Didn't Southern also withdraw the 313s used on coastway services about the same time, so they lost nearer 200 vehicles.
Some shuffling of 375, 377 with South Eastern took place, and there have been some 387s moved (from GWR and Great Northern), so Southern have already regained some of the losses.
However unless GBR sticks to rigid pre-nationalisation boundaries and workings, in theory there is no reason why services to places like Dorking or Horsham (from Victoria) couldn't switch to 701s if it helped balance out stock so other units can move to where extra capacity is required.