CAT5 is essentially dead. Highly recommended to use cat6/e as a minimum, or cat8. The world is beginning to switch to multi gig ethernet and CAT5 is simply insufficient for that.
Yes it will work at gigabit speeds and most things you do will not require more than gigabit but who knows what we will be running in 10 years and cat 6 can handle 10 gig over a pretty good distance which should be sufficient until it needs to be completely replaced.
That being said, unless you are currently running a multi gig ethernet setup and are running into bandwidth limitations on CAT5 or cat5e, there is no need to pull and replace what is already there. This advice is for new deployments.
I agree that 2.4 gigahertz is ultimately doomed, but we are easily 25 years away from moving out of that space and even then there will still be use cases for it.
If you were to suddenly disable all 2.4 GHz Wi-Fi connections across the world a large portion of the world would be stranded without Wi-Fi.
And since smart home devices and many other products that are actively being created required 2.4 gigahertz to function, any router that did not include 2.4 gigahertz would be e-waste before it was even taken out of the box.