I'm a bit confused (some might say that's not too hard)...
Are you saying that if the car is written off in an accident in a few years time you want GAP to pay the difference as to how much you paid for it now?
Oddly that is exactly what GAP to invoice offers, for less than a couple of hundred pounds.
The question is "How many cars are completely written off?" GAP doesn't cover the depreciation of a car that is damaged and repaired for example.
So GAP insurers can offer this insurance cheap because it doesn't happen often.
The trouble is that what you haven't covered is "How to pay for a replacement car in x years time". So most people would not consider GAP because they are already considering how they would finance the next car in X years without GAP insurance.
GAP makes sense if you are at the edge of affordability, where that couple of hundred quid can avoid you having to seriously downgrade a car because you can't afford the replacement costs.
Beware that GAP may not cover the entire invoice cost though, not all extras and costs are necessarily included.
To the OP, cash buyers have probably never considered GAP. As far as I am aware it is only recently become a popular insurance, coming to the surface as part of the personal leasing market.
I didn't think it offered good value for money, but then I last wrote off a car 30 years ago and I'd only paid £900 for it about 2 years before.