I'm not personally seeing any immediate need for anyone to upgrade routers if they're planning on getting the new Galaxy S10, and that's if the new phone comes with WiFi 6 support.
For one, routers that support WiFi 6 at the moment cost around the $350 to $400 range, which is pretty expensive. And the benefits of WiFi 6, while great, may not be quite worth a forced upgrade if you already have a good WiFi 5 (AC) router.
I have a 200Mbps internet plan and I'm using a Google WiFi mesh system that uses the AC standard, and I have no complaints with the 30 devices I have connected to it. I can easily stream 4K videos on two different TVs while others browse the web or play an online game.
Plus, despite better congestion management, WiFi 6 routers may not make sense until you also upgrade other devices around your home, like smart TVs and smart home devices.
And even if you frequent congested places, you'd need to hope that these locations also upgraded their routers to WiFi 6 models for your WiFi 6 devices to take advantage of.
Still, it's worth checking if any new WiFi-connected device you buy in the near future supports WiFi 6 should you want to upgrade to a new WiFi 6 router down the line.
If you're rocking an older WiFi 4 (N) router, or something even older, the WiFi 6 generation would be a good standard to upgrade to. WiFi 6 routers will be backwards compatible with devices that use older WiFi standards. While you won't enjoy WiFi 6 benefits on devices that don't support it, you'll likely enjoy faster speeds and better range with a WiFi 6 router than a WiFi 4 or older router.