With all things, I think it depends.
Road bike technology really hasn't changed that much in the last 15-20 years. Carbon has become more ubiquitous and we went from 9 to 10 to 11 and no 12 speeds. We also went from Triples to compacts and now in many cases, single rings. Compacts are definitely an improvement over triples - which never shifted great. And many people like the ride of wider tires and rims, which many older frames don't accommodate. So there are some aspects where things are outdated.
For cross, it's all gone disc. And if you are considering gravel, that's all disc too. So, why would you want to buy something that isn't disc compatible? Non-disc cross bikes have lost a lot of value. If you are looking for a bike to commute on or ride around the neighborhood, a canti-brake cross bike would probably be a good value.
With TT and Tri bikes, they continually create new and more aerodynamic options. If your goal is to go as fast as possible, newer is probably better.
On the mountain bike side of things, yeah, the technology changes a lot. Today's suspension systems are so much better than what was available 5 years ago. And shocks have a lifespan. As they get older, they are increasingly costly to repair or replace.
But if we were to look at something like track racing, there has been almost no development in new products. They still run narrow tires. You can buy a 15 year old wheel and frame and be pretty close to what is at the top of the line today.