How long Can iMac Retina 5k 27 (2014) survive without Updates?

Quite a few years, if you keep your browser up to date & are careful about what you do on the internet, especially stay away from certain types of websites, be very cautious about pfishing and other social engineering and don't install any pirated software (for that matter, also be careful about where you get your software). You might also consider adding Malwarebytes for some additional protections.

As @BDAqua already suggested, install Firefox or Brave and keep it up to date. I have used Firefox for many years, many releases and it has always been stable. Firefox already supports HTTPS Everywhere and for a bit of extra privacy & security I add Privacy Badger & uBlock Origin plugins. I also make it a practice to have my browser automatically clear all cache & history when I close the browser. For future planning, you can expect Firefox to be fully supported on Big Sur for at least 3 more years. Personally, I don't care for the Brave browser because it contains code from Google. Although it's not Chrome itself, it still has Google code inside ... and I don't trust Google.

Be aware that as time goes on, some apps you use will no longer be updated on BigSur. That may or may not be a problem for you. Also there are, and will continue to be, apps that cannot be installed on anything but the latest 1-3 versions of macOS. In that regard, Big Sur is already outside the window.

We can only guess bit I'd say you have 2-4 more years... if you get a better Browser than Safari, which gets outdated by Apple not updating outside of newer OS versions usually.

Mostly you just need a more modern Browser than Safari on Catalina, Mojave, or High Sierra like Firefox or Brave.

https://brave.com/

Opera…Thanks

https://www.opera.com/download/requirements

Opera system requirements | Opera

Thanks to dialabrain…

Firefox 115 is now the current version. runs in High Sierra.

119.0.1 is now the current version and requires 10.15 or later.

https://www.mozilla.org/en-US/firefox/new/

Thanks to A Brody, Browsers by Mac operating system…

Browsers by Mac operating system - Apple Community

TelfordPrinter wrote:

I understand that the new OS (Monterey) is not compatible with my Mac and there will be no more support for Big Sur.

How long will my Mac carry on functioning, it is in excellent condition and works well and would be criminal if I have to ditch it and buy a new one

You've got it backwards. If you have an older Mac that can't be upgraded anymore, then that computer will be locked into that operating system. It will continue to run just as well as it always has until the physical components start to degrade.

Had you been able to upgrade, then the computer would get slower and buggier every time you upgraded. It's true that you won't get any new features from new versions of the operating system, but presumably you were happy with the features you had before.

The only real risk is if you buy some newer device like an iPhone or new Computer and use some new feature that causes your iCloud account to get upgraded in some incompatible way. Usually there will be a warning about that, however.

Heh heh ... just last night I fired up a 2004 eMac running Panther for the first time in 6-8 years. It was a little cranky at first, but I was able to copy off a half dozen .cdr image files that I had left on it years ago when I was making DVDs. 20 year survival, not bad!

I am also still using a 2012 MacBookPro running High Sierra as my travel laptop. It works perfectly fine. I don't keep any private or financial information on it and I only connect it to the internet in my hotel or at family members' homes; even then I am only checking email, Skyping, downloading a boarding pass or doing a bit of browsing.