Some WiFi routers can be configured to support multiple standards on multiple bands but sometimes do not reach their peak performance when operated that way. Someone with a new high-performance router would probably look for new devices to support that level of performance.
It’s true that for many homes their current WiFi router may not be the latest and greatest, so buying HomePod without the latest WiFi standard is not an immediate problem but keeping up with the latest WiFi standards on new (and arguably, expensive) products is all about
longevity.
With these products, Apple is deliberately delving into planned obsolescence. They expect customers to treat these devices as semi-disposable and be replaced every couple of years, much like iPhones are. This is further evident by the omission of an analog input. You can’t even choose to use a HomePod as a plain old amplified speaker after it is relegated to “legacy” status for feature updates (
Or when it’s no longer compatible with your shiny new WiFi router 
).