i've had to look at my nat gateway zero times since i set it up a couple years ago. i can't say that about any VM host i've got. to me, that's easily worth the few dollars a month that aws charges for it. it's cheaper than hiring somebody, and it's cheaper than me.
That said, the paid NAT gateways do also publish metrics. That can be nice when debugging a legitimate issue, such as when your gateway actually runs out of NAT ports to use.
The market will provide. In this case by increasing prices to the point of maximum value extraction from people who don't want to deal with all that. There's a high initial cost to moving to something else here, with a lot of people dragging along paying more than what the market would otherwise equalize to, out of avoiding that initial hurdle. (And long term commitment of a resource, of course, one with low average but indeterminate excursion cost.)
2) Companies will happily pay thousands in recurring fees for the built-in NAT gateway, but if an engineer asks for even half that as a one-off sum to motivate them to learn Linux networking/firewalling, they'd get a hard no, so why should they bother?