The 9s is going to be a good phone for 4+ years because its going to have independent rom support from modders. That isnt the case with most other chinese based phones(or even korean ones, since they lock down the bootloader)
If you're on RFD cause you're cheap(and lets be honest, we are. We want to get the best value for our dollar), you'll want something that will be updated for years, because in all likelihood software will be the first thing that'll force you to update. Even a bad battery or a cracked screen can be limped through for a few years, if the OS is kept up to date. That isnt the case if you're stuck on, say, marshmallow in 2020.
Even if Xiaomi drops support after 2 updates, theres still going to be a bunch of people working on roms probably till 2025.
I bought my last phone(zuk Z2) cause of that. Low and behold, it was one of the first phones to get android 11... from Lenovo? Nope, lineage modders, cause its using a common chip(SD820) on an unlocked platform with a bunch of users to make doing the port worth it. That'll be the case with the 9s. That definitely WONT be the case with the sketchier brands like umidigital or doogee or possibly realme.