when should i look at replacing my macbook pro late 2013? i currently have macos Catalina running on it and could upgrade it to big sur. I'm unsure if I will upgrade to big sur given the issues I've seen with big sur and my hardware. after Catalina is EOL is that when I should look at upgrading or can I keep using it?
There are probably many people have stayed on the macOS for reasons like software wouldn't run on the newer version like Big Sur or even Catalina. But in your case, hardware is more difficult. With software, there's always a chance that a vendor may change code to support the newer version.
Some hardware vendors will figure out how to make it work but others will have to wait on collaboration with Apple to do so (e.g. 3rd docking stations that use display link software). I think I stayed on Yosemite a good while, finally just gave up on the docking station.
The big issue with staying with the current macOS is security bugs that will no longer be updated after the EOL. After a while, the next update will always be the next OS.
While you can probably remain functionally problem free, I would say that upgrading is something to consider especially because of security patches.