The problem is it's not just about time. It's also about being confident that you'll get a comfortable night of decent quality sleep. If I've got a day of meetings in Glasgow, I can't afford to be exhausted all day because of problems with lights, air conditioning, temperature, water systems, lack of food, etc.Some facts. For euston to glasgow. Where it's delayed, its fairly consistently less than about half an hour.
https://www.recenttraintimes.co.uk/Home/Search?Op=Srch&Fr=London+Euston+(EUS)&To=Glasgow+Central+(GLC)&TimTyp=D&TimDay=8p&Days=Wk&TimPer=10w&dtFr=14/04/2019&dtTo=23/06/2019&ShwTim=AvAr&MxArCl=50&TOC=All&ArrSta=5&MetAvg=Mea&MetSpr=RT&MxScDu=&MxSvAg=&MnScCt=
The sleeper is more expensive than a day train plus hotel (and the last time I looked, the sleeper was £100+ more expensive), so to be justifiable it needs to offer me everything a hotel would, and something extra on top. At the moment, it doesn't do that.
I'd consider using the Fort William portion just for the experience, but for a journey to Glasgow / Edinburgh the sleeper, for me, just doesn't have a purpose at these prices.