Venture update not available for my iMac (Retina 5K, 27-inch, Late 2015)?

Not compatible, sorry: macOS Ventura is compatible with these computers - Apple Support (MY)

scotjimland wrote:

I should have added, why not..

Because the hardware to support all the new features in Ventura didn't exist until 2017.

and is this another Apple ploy to make us update a perfectly good machine?

See my previous comment.

The Late 2015's are maxed out at Monterey (macOS 12).

You can certainly continue using your iMac on whatever version of macOS you currently have, and upgrade it to Monterey if you wish to. Monterey will continue to be supported with updates (including Security Updates) for at least another 2 years. Your iMac should still be fine even after that.

.. and is this another Apple ploy to make us update a perfectly good machine?

Upgrading macOS is never required.