Sure - I realise that.
It's perfectly possible to do a self catering holiday without driving, but people will do it, as long as it is more convenient and not loads more expensive than alternatives.
Motorail makes taking your own car somewhere far away a little less damaging in environmental terms than it would be otherwise (maybe). But that's all.
From a customer point of view, of course it's an attractive option.
From a practical point of view though it must be near impossible to deliver given the size of the average family car these days. Unless the motor-rail stock was open a bit like the freight shuttles on the Channel Tunnel, putting a conventional rail body around today's SUV type vehicle and having any room for opening doors etc would make them just to wide to fit the network. If new stock were built it would have to plan against the potential increase in vehicle sizes over the next 20 years otherwise it would be an option open only to a diminishing and niche market.
From an economic point of view, I can drive London to Scotland on far less than £100 worth of fuel. I've got all the other costs anyway so the only advantage is fuel cost saving. Given each seat as an Advanced Standard LNER London to Edinburgh is £74 or thereabouts, it's vastly cheaper to just drive if there is more than one of you.
If a motor rail coach is 22m long, you'll carry at (very) best 5 cars, more likely 4. 4 Families. Maximum rake of 16 coaches would carry what, 40 cars / 160 people if each had 4 people in. Reality would be a much lower ratio of people/cars. Even at 4 per car and they stayed in some sort of cabin thats 20 people per coach / 2 per cabin / 10 cabins per coach. Thats less than the current sleeper.
The economics of a motor rail service would horrific as the operating costs would be little different to a sleeper service with the added liability for vehicle damage.
If it were possible, the cost to use it would have to be hugely greater than the alternative of just driving or the level of subsidy eyewatering. And to what end? To capture a minuscule fraction of the overall tourism market. Scottish Government would be better advised to spend half the money on giving people free flights and hotel stays as an inducement to come and letting them spend in the economy when they got there.