If you don't have a long range battery, you might have to stop every 60-80 minutes of driving to charge. That will get annoying pretty quickly, especially since it's not like a smaller battery takes all that much less time to charge.
So I would expect the long range battery market to be quite big, and a range extender would have a good chance at grabbing many of those users.
Even if gas stations are somewhat uncommon, you'd only need to find one every 200-300 miles, and the worst case scenario is just that you go back to pure electric mode. You could also fill up a 5 gallon can for another huge chunk of extender range.
A single tank could save you an hour of waiting to charge.
To get "gas ranger extender" you still need oil drills, tankers, pipelines, refineries, tanker trucks, gas stations.
Without gas cars, we can retire all that infrastructure.
Well, we can obviously reduce it if we reduce gasoline consumption from the status quo (true with moving to hybrids, especially plugins using gas mostly for range extension), but since its not used exclusively for autos, we can't retire it even if we replace all autos and do nothing else.
I feel like the population that both drives very long distances regularly and is willing to spend a large chunk of money of make the trip a little faster is pretty small. Plus it would be pretty hard to find gas stations if most people drove EVs.