It almost always seems better to solve the structural problems of the mainline carrier than to start a new airline. That seems harder ex ante but given the track record of ‘creating new airlines’ and walking away from economies of scale in the process the airline within an airline model hardly seems the uniquely better option.
Indeed, British Airways has already altered its short haul product to be more like a low cost leisure carrier, with less seat pitch than Ryanair (even in business class). They’ve reduced their labor costs through a series of contracts, B scales, and labor actions over the past dozen years – a process which accelerated during the pandemic.