I didn’t exactly win the lottery with my i5-2500k, but I can still hit 4.5GHz on all 4 cores with air cooling. Even 2nd-Gen Ryzen doesn’t boost that high. I admit that it probably isn’t hard to squeeze better overall performance out of a Ryzen-based system, but like you said, it’s a matter of replacing more than just a motherboard. Even if you can find great deals on CPUs and RAM, all that new hardware will still add up.
I also think waste should always be a consideration here. Ultimately, if you’ll get more years of use from a more modern upgrade, then it might be less wasteful to go with a new mobo/CPU/RAM. But if that isn’t necessary, you may as well go with the most minimal replacement possible rather than throwing a bunch of functional hardware in the trash.