"Just for fun" last night I hooked up my 2012 15" Macbook Pro to a new (new to me) external apple LED Cinema Display monitor and got online for a bit. My 2012 has been upgraded with an SSD and 16 gigs of memory. Honestly, it was just fine without any issues. I was running Mojave, Catalina and Windows 10 with it, triple boot, and was able to update to the latest versions of Skype, Zoom, Firefox, OpenLibre Office, Keka, and Thunderbird in each OS and everything was nice and fast...I literally had zero issues in any of the three operating systems I ran.
I could easily use this plugged into a monitor as my main machine without "feeling like it was holding me back" for what I do. Maybe you need a speed demon of a machine for some weird esoteric work you do for for daily normal uses you don't.
MY definition of "it's too old to be usable" is when Firefox and Thunderbird are no longer officially supported with updates to the current version, and this model is still fully supported by both of them. Web browsing and e-mail has really become most average peoples usage of a computer I'd argue.
I guess it depends on what you want to use your machine for but unless you have money burning a hole in your pocket or like to upgrade for the hell of it, what's the big deal with your current setup?
The longer you wait to upgrade the newer and faster of a machine you'll eventually get!
Good luck!