I used to have a 4790K, and for the approximately 5 years I had it, idle and load temperatures did not change, at least not enough to make me notice it, although I'm pretty sure I didn't see any increase at all.
I did use an aftermarket cooler though, so I can't say if the stock Intel cooler could have become less efficient due to wear on the fan bearings for example, but I doubt it to be honest.
Other factors could have played a role in the increase of the idle temps you've experienced. Both ambient room temperature, or swapping out hardware for something which creates additional heat (like the graphics card for example) that could affect the CPU cooling performance, could have contributed to the rise in temperature.
Fan speeds could be different, possibly caused by changes or updates made in BIOS. Dust build-up in case fans or dust filters (if present) could have reduced airflow. A worn out cooler perhaps
But if the CPU has been running at stock settings the entire time, and has been cleaned for dust regularly, the thermal paste is good, and no other changes has been made at all to the build, I have never personally seen the CPU temperatures increase due to CPU degredation over time, especially not over just five years timespan. That doesn't mean it is not possible, but it is not something I remember to have come across elsewhere.
But if certain hardware changes has been made, or if you've moved to a different room, ar maybe even a new town, or if ambient temperature has changed, or case airflow, or comparing to a different season in the weather, those are all potential causes for the changes to CPU operating temperatures
If you can't think of any changes at all, I don't think can think of something else which could have caused it. So you never know - sometimes hardware behaves outside of what is usually seen or expected.