This is a serious issue that many of us who organize races are dealing with. Road racing on a whole is on a slow decline. There is a clear shift toward crit racing, particularly among younger riders. This is even more pronounced in time trials (as your chart illustrates) than road races. The financial cost of putting on a race is substantial, and break-even or large losses are so dependent on the weather. I don't really have an answer about how to make time trials and road races engaging again, but I feel like we have a niche in the Wintergreen Ascent and I believe it can continue to grow with good marketing. I think people want an enjoyable experience before/after the racing part, which downtown style crits are able to provide. I've been to plenty of road races where there was nothing to do afterwards but go home. I think that's one reason why gravel is so big.
The other possibility in achieving financial sustainability is less about sponsorship (which, as you say, will be difficult to get when you only have 100 riders) and more about lobbying the local government to offset your police/EMT/traffic control expenses as in-kind contributions.
Ethan