Is Sierra the most modern version your machine can run? If so, it is time to start looking for a new or newer machine ... Sierra is officially three versions back, and as of next month sometime it will be four versions back ... that translates to no security updates anymore. I am of the opinion that is better to keep software (all software, not just Apple’s) as up-to-date as possible, since each new one fixes bugs and closes security issues. That said, you don’t have to upgrade on day one of release, but within a month is usually a good rule of thumb.
There are times when such a big change happens (like the move to exclusively 64-bit software as occurred with Catalina) that old, abandoned program will “break” with the new release, and in those cases it is okay to wait for a while (a short while, i.e. less than three years) before upgrading the OS while you upgrade the software you need — or, if no upgrade is in the offing, while you search for a replacement/alternative.
Though I have a new MBP that runs the latest software, I’m in that boat with my older 2012 MBP — it CAN run Catalina, but there’s some software on there (a few games I don’t quite want to give up yet, even though I very rarely play them, and some audio software I use for the Space Javelin podcast) that never got upgraded enough to run on 64-bit only systems. I will over the next year or two have to find a replacement or just give them up.
I should mention that there is one exception to this — if you really want to keep an obsolete machine around for whatever purpose, at least take it offline (not on the internet) so that the main vector of attack is not available. I have some musician friends who have old computers they don’t have connected to the net and use them as dedicated (almost) single-purpose machines to run the unsupported software (or the version of the software) they prefer.