To be fair... as the owner of an S10+, I'm not particularly seeing a reason to upgrade, and I'm not convinced that's likely to change in the run up to the March 2021 release date.
Camera: looks interesting, but I'll wait to see how successful the pixel binning is - and the folded optics on the Ultra
Screen size: the S10+ is already at the limit of how big I want a phone to be; once you slap a protective case on[*], the S20 Ultra is going to be ridiculously big
Screen resolution: meh. it's the same as the S10+ - and I ended up dropping mine down to 720p, in an effort to save battery life. Hasn't really made any visual difference - but then, it's seemingly not really improved the battery life any, either...
RAM: what's the point? Looking at my phone, it's using 4.1GB of ram, of which 2.5GB is flushable cache. Any RAM above the 6GB limit (unless playing video games, maybe) is just going to sit there and waste electricity!
And then there's the headphone jack, or lack thereof. I like my wired headphones - they're cheap, easily replacable, don't require recharging and aren't fitted with tiny, non-servicable batteries which are likely to fail completely after just a year or two [**]. And I'm not that keen on having to carry around a USB-C adapter; it's something else to lose or break and makes it even harder to safely cram this massive slab of expensive technology into a pocket.
So, my current verdict is a resounding "meh".
Still, who knows? The 2-year contract on my S10+ will run out around March... I might give it a few months for the price to drop (and for the reviews to come in) and then go for one!
[*] I learned the expensive way to always keep my phone in a case, after an incident with a caseless S7+ several years ago!
[**] Apple Airpod batteries are expected to have a 2 year lifespan...