The Ultra is, far and away, the most significant upgrade in Apple’s watch lineup ever. Hands down, so to speak. Nothing even remotely compares to the S7 -> Ultra change
As such, you can bet your bottom dollar that the next version of the Ultra will be a trivial upgrade by comparison.
So, if the Ultra isn’t exciting enough for you to want because you want to wait for something even better a few years down the road … you’ll never stop waiting.
Two suggestions, then: get an Ultra and plan on wearing it until it dies, knowing it’s going to be an amazing watch that should still be going strong several years from now. Maybe if there’s a truly radical upgrade, or some minor upgrade that’s enormously significant to you (such as blood glucose monitoring if you suffer from diabetes), then get the upgrade; otherwise, just enjoy this truly glorious piece of technology. That’s the path I’m personally on.
The other option: budget $10 - $20 per week for a watch upgrade fund, and spend it every year or two on the latest-and-greatest.
(Your other option, of course, is to never buy any new technology. That’s a perfectly valid option, and it’s the one you’re implicitly choosing already. I’d just suggest being upfront with yourself if that’s the case so you can let go of some of the angst.)