Personally I think both governments would scrap them in a heartbeat if they thought they could get away with it.
But I think in the context of an extra few hundred million here or there to sort out whatever today's disaster is, it's just not worth the political capital to have weeks of argument with well organised lobby groups (that's us folks), to save a few million here or there.
Speaking from Scotland perspective, I think when CS' new stock came in there was embarrassment about the whole "£400 one way for a Bed" thing .... from a socially liberal centre-left government.
However, whatever the reasons and merits are, they have moved away from it being a "lifeline service to London" from various West Highland Line halts (the defence in the 90s when they tried to lose the Fort William), and are now more honest about it being a tourist trap to bring tourists to Scotland (their value to the highland economy really cannot be underestimated), and based on the number of times I cannot book said £400 room weeks in advance, where only the seats or one class of cabin is available, they are clearly doing something right, as it is frequently full.
The bigger question is when the Riviera stock is genuinely life expired and there is a need for further significant investment.
But as we have discovered and are discovering, the old line of "once HS2 is built, this won't be necessary" is easier delivered in an animation launch film than with a tunnel boring machine and a shovel (and an even weaker argument in Penzance !). Personally I am skeptical I will be able to achieve either of the following things in the remainder of my working life (20 years or so):
1. Get an early first WCML train that arrives significantly before the current time (0915 or so).
2. Get an early first WCML train that leaves significantly after the current time (0430) and arrives at the current time.
And so for various reasons of my work, and a 3am alarm call to get to said train having much the same impact on my day as not sleeping at all, the Sleeper will still have some market, and I for one am glad of its existence.
I imagine there are things they can do which will go far more under the radar and save a lot more money (removing large amounts of "excess" rolling stock from many of the franchises in the country, for example).
Given there is some merit to the argument that the primary alternative market for the sleeper is airlines, rather than day trains (said well heeled travellers being time poor presumably resistant to Avanti's penchant for depositing Scotland bound travellers in Preston or other English palaces of Rail at all hours of the day or night), it would be a brave incoming government who would save a fairly small amount of money for the "Government slashes electric rail services to cut costs in boost for Domestic Air Travel" headlines.
Governments of all flavours will go to almost all lengths to avoid bravery.