I completely understand why people get infuriated by the sheer rise in people opting to travel around the country in either motorhomes or campervans (handy guide to the difference between motorhomes, campervans etc. here -
https://www.brownhills.co.uk/campervan-differences.html) and of course the inevitable irresponsible/inconsiderate behaviour and poor driving/road hogging this can bring.
To suggest that all or most folks who choose to explore Scotland (and elsewhere in the UK) this way are a problem is shortsighted and lacks nuance. I think KeithS has covered most bases re infrastructure and the role rental companies play, and as others have said other road users can be just as bad. Dangerous overtaking, inconsiderate or aggressive driving from all forms of road user is possibly a bigger issue generally, though I can understand why motorhomes cause anger when in high volume. Improved education re one-lane winding roads and that remote areas are not playgrounds but part of communities and nature and there for should be respected is vital as is improved infrastructure.
I find it pretty saddening that people are often viewing everyone who has motorhome or campervan as selfish and irresponsible idiots. We own a van that is part converted into a campervan. It is a short wheelbase low top Renault Trafic (single "f" intentional). It is actually a bit shorter than our Ford Mondeo estate to give you an idea of size. So it fits into a standard parking space and can be driven at speed. We park responsibly and always research possible places to park up for the nights we intend to stay in the van rather than accommodation, as we don't want to cause upset or park inappropriately. We very rarely see a queue of traffic forming behind us and in fact have been subjected to being stuck behind motorhomes getting increasingly irritated by them not pulling over to let the traffic flow so I do understand that. We like to use local shops, cafes, and pubs while away - though recently we have done this a little less due to Covid-19.
The NC500 situation in bonkers. I don't know why anyone would want to start encouraging high numbers folks to do this route in large vehicles without appropriate infrastructure. My former partner and I basically followed this route back in 2008/9ish I think, way before it became an official thing to do because it seemed like a beautiful road trip - which of course it was, and that was partly because it was pretty quiet. We were just in a car with a tent though and stayed at campsites and the odd budget B&B which worked for us at the time. I have little desire to repeat it these days given all the problems it has created and, due to the popularity of the route, it would likely be a frustrating experience!