When a government makes a deliberate decision to open up a profitable part of it's publicly owned monopoly postal service to competition, then they have made a deliberate decision to saddle the taxpayer with subsidising the remaining less profitable part: unless, that is, they can get another bunch of investors to take that obligation on for them (no subsidies).
It isn't a question as to whether tax payers should pay for it, that was already committed to by the government that chose that path on behalf of the public. I think this was an example of the utter foolishness of the government of the day; but the public gets the politicians they vote for. The rest of us have to put up with the majority voter decision.
If the private investors that bought RM are now complaining that the obligation is at risk (which presumably can only mean they have made an appalling investment and are likely to go bankrupt) then there is little that can be done to continue to safeguard a vital service to the public. If they wish to take the loss and hand it back then any decent government has no choice but to take the responsibility as no one else is going to invest.
In that case, as a public service it has then to be funded publicly: but it will no longer have the ability to cross subsidise from the more profitable areas that the government of the day effectively passed over to investors.
Unless, of course, a government had the courage to close the market once more and return to a public postal monopoly and sanity. But that's unlikely. Folk are keen to have a choice in parcel courier now, so have to pay the price in funding the less profitable remaining mail service.