Sure, if you're only talking about a single device like a thermostat, it makes perfect sense to replace it with a HomeKit supported model if it's that important to you.
But of course, if you only have a single device like a thermostat that is already supported by a reasonable app like the Nest app, then why bother lifting a finger or spending a dime to move it to HomeKit.
I have the Starling hub and through it I can access my Nest thermostat, Nest Doorbell and multiple Nest cameras.
Frankly, having it integrated into Apple Home isn't all that important even with all those devices, as there isn't much added value relative to the native Nest app. But I was curious to see how well it would work so I grabbed one and plugged it in. Works like a charm.