JL wrote:

I really wouldn't be surprised to see the Big 4 returning

Which, ironically, is exactly how John Major envisaged privatisation -
before he was talked out of it.

was wondering why Govia don't merge South-Central with South-Eastern
as Virgin have done with VXC/VWC quite sucsessfully for 11 years;

Virgin haven't merged the franchises. It's not within their power to do so.
The DfT will decide if and when franchises are merged.

Virgin West Coast is run by West Coast Trains Limited. Virgin CrossCountry
is run by CrossCountry Trains Limited. Both are accounted for independently.
However, they are marketed under the common trading name of Virgin Trains.