I don't know what service you use, but if you sign up for Google Fi today, the Pixel 3 XL is 50% off, $449-499 depending upon storage (the Pixel 3 is also 1/2 off). I'm sure they are clearing inventory before the Pixel 4 comes out in a month or so. The Pixel is probably a better phone than the S9. I don't know how long you need to stay on Fi for in order to be qualified for the deal. If you're travelling internationally, Fi is pretty great. There are cheaper MVNOs, and you're dependent upon T-Mobile, though.My S7 is slowly becoming worse and worse, is the S9 in a good place still for $499 unlocked (I travel a boatload for work so hotswapping sims is key).
If Samsung is your thing/you don't want to change carriers, new S9s are going for like $400 on eBay, and as low as $250 or so used on Swappa. If you're working off an S7 right now, I'd imagine you'll be fine with an S9 for 2-3 years depending upon your use case. Outside of the cameras, phone hardware has really only incrementally improved over the past 2 years or so. If it weren't for battery deterioration, I'd still be on the Pixel 1.