I hate to be a bore, and regurgitate the same “leftie” discourse that gets repeated a lot online, but:

Royal Mail would become “financially and operationally unsustainable in the long term”

This is because Royal Mail has become private and is now required to become profitable / increase it’s value over time. Which is nonsense. It should be a public service funded by a mix of direct payment (e.g. stamps) and taxation.

If continuing to run six days per week, as the are currently obliged to, has become unsustainable then perhaps it is time it returns to public ownership.