My house was built before CAT6 came out. It should be easy to upgrade as everything runs through in-wall conduit. I figured I'd do that for future-proofing.
I've not really seen a need to upgrade. It works well enough and there are jacks in most rooms. I also don't have the bandwidth to make it matter much, though fiber will be here before too long. Upgrading then may matter. (Fiber wasn't going to come up my road, but I contacted the company and a couple of neighbors and I will pay for the fiber to be run.)
Cat5e should still be fine. I'm not going to bother paying for full GB service, as I don't need that much bandwidth. I'll be fine with half of that and CAT5e should be viable.
This is about the 'inadequate' comment you made.
I could see it being inadequate for some people. As for me, it's still holding up and still fit for purpose. I've lived in a bandwidth-impoverished area for going on two decades. So, my needs, perhaps better said expectations, are different. I don't even see a reason to go full boar on the GB speeds. As I am technically a business, they say I can request even higher speeds. I'm simply not interested in that.
Then again... We'll see how I feel after a few months. I may end up wanting to splurge and increase my speeds. It could happen.