Speaking as someone who was a regular user of the sleeper under Scotrail, I'm not sure that it was a mistake to separate the franchises in 2015. Equally I'm not saying that it would be a mistake to bring them back together in the future.
The problem with First Scotrail (and National Express before them) was that they had no management focus on the sleeper being as it was so far removed from the core operation. I'm less sure about NEX, but I know for a fact that First regarded it as a damned nuisance. Their 2014 bid for the sleeper franchise pretty much said that, and certainly didn't include a proposal for new trains.
Serco was as keen as mustard when they started, with managers all over everything and massive improvements in hospitality but, sadly, not a clue about the railway. This cluelessness continued with the specification and build of the new trains, delivered massively late and incurring huge losses through the need to keep the old stock going way longer than expected.
Setting aside operational resilience, which I agree is an important part of the mix, let's think about where we'd be now if the sleeper had been held within the Scotrail franchise in 2015. We have to assume that the presence of the sleeper wouldn't have been enough to deter Abellio from bidding in the first place.
We'll never know now, but the chances of them having thought enough about it to propose a new build fleet would've been very slim. So the mk3 sleepers would've joined a very long queue of rustbuckets at Doncaster. They might still not have been done now (and that's being charitable) and would've been most unlikely to have been converted to have ensuites. Arguably this work would've created a common fleet with the HSTs of refurbed and compliant mk3s, but with them all in urgent need of scrapping all at the same time in another few years.
There would've been as big a flourish at the start as there was the daytime trains. In other words, no flourish. Bad food, bad bedding, and bad service from crew who would've hated Abellio even more than they dislike Serco. They would've run it in the same dim witted penny wise pound foolish way as the rest of the Scotrail network and everyone would've been up in arms, just as they are now, but for different reasons.
At least now we have a fleet of brand new trains offering modern standards of accommodation. Okay, there are a few avoidable problems like build quality, badly laid out cabins and pantries, uncomfortable seats and knocking bogies, but none of those are as insurmountable as vehicles way beyond the end of their design life. Whoever runs the sleepers, Scotrail or a separate franchise holder, and to whatever business model and fare structure, will have that stock available to them in the future.
Or maybe I'm completely wrong and Abellio would've proposed new trains which would've been built perfectly within 18 months by someone other than CAF.