As I understand it, Serco won't be in any hurry to hand back the keys. They might be losing a lot of money now, especially through nursing the old stock, but this won't continue for much longer.
https://www.transport.gov.scot/medi...t-caledonian-sleeper-23-may-2014-redacted.pdf
The Franchise Agreement through a profit support adjustment (p309 of the Agreement) protects them against losses incurred after the fifth anniversary of start of operation, in other words in around a year's time.
So if the sleeper's losing more money than expected one year down the line from the introduction of the new trains then it's the taxpayer's, and not the operator's, problem.
If Serco's business model misfires and losses increase then that's something that'll need to be played out in the political domain, but don't expect the operator to have too much to say about it.