and the fee was a "suggested donation" (this gets around the professional driver legality which is killing their model), then I'd be a regular user
In this country, accepting payment for driving is "driving for hire". Regular vehicle insurance is for driving for "social, domestic and pleasure" purposes ("SDP"), which includes getting to and from work, but does not generally include driving as an essential part of your employment. So, you're OK for commuting to and from work ; you're OK for driving to the restaurant for lunch ; you're probably OK if you have to drive to the offices on the other side of town for a meeting. But if you drive around 3 clients/ suppliers collecting and dropping off materials for 3 hours a day, you're probably driving outside the terms of your insurance policy. Which means that you're driving un-insured.
Obviously, there are issues of verification. But the more driving you do, the more likely you are to fall outside the "SDP" categories.
There are a lot of companies who do things like hiring housewives to spend the time between school runs doing parcel deliveries in their cars. And there have been several cases of such employees (technically they're self-employed sole traders, individually responsible for their own companies following the laws) having a crash with a pile of parcels in the car, and this being spotted by an investigating police officer.
"Are you delivering these parcels for 'YouDeliveryCo', Mrs Bloggs?"
"Errr, yes. Is there a problem, Constable?"
"Can I check your insurance details, please?[Few minutes phone calls]I'm arresting you for driving without insurance. Your insurance doesn't cover you for driving for hire."
If your proposed "suggested donation" system started to happen, then you'd see exactly the same situation happening again.
Yes, there is an implication of the above. If you share a car with a buddy for the weekend to go walking/ cycling/ away/ whatever, you're skating on the edges of "driving for hire". And it's true : you are skating the edges. If it is something you do every so often, then it's just not going to be detected. But if you're doing it often enough to make a living at it, then you're almost certainly going to have enough crunches, grinds and scrapes that your insurance company is going to notice. And yes, the insurance companies do talk to one another, and yes, you did agree to them doing that when you signed up for your current insurance policy. (If you scored the condition out, they'd decline the policy. And they talk to each other about declined policies too.)