As you may see from some of my other 'iMac' posts on this forum, I have a different take on this compared with many other users. You don't say exactly what model iMac your parents have, but as it's a 24" and 10 years (or more) old I presumed it was an early 2009 model iMac9,1, that being the last year that the 24" was produced. However, natively that model only supported up to 10.11.6 'El Capitan'.
As you say it's running OS Sierra, either someone specifically patched it to run Sierra (which I doubt) or it's a 21.5" iMac10,1 or 11,2 Can you please confirm which model it is.
Anyway if your parents need is for light use only i.e. will use "just MS Office apps", do little or no photo or video editing, also probably use it for internet browsing too, I cannot see the need for forking out money for an M1 silicon machine as some have suggested - although I agree they appear to be very powerful, popular machines.
I am still puzzled at your comment, "10 year old or older iMac and it runs sierra which is nearly worthless these days".
If you meant Sierra is nearly worthless today, I disagree, and if you refer to the iMac in general as nearly worthless I totally disagree, but guess that depends on where you live. If in the US, and if these circa 2009 iMacs are being dumped as 'worthless' that's sad, as I can assure you that with a $50 SSD added these Core2Duo iMacs are capable of performing reasonably well for a number of years more. Here in Europe a well spec'd out 2009 21.5" model running Sierra with SSD and with 8GB or more upgraded memory are still selling for up to 300€, so yes they are still considered as being quite capable machines for word processing and similar tasks and web browsing.
However if your parents think a machine that old should still be able to do everything - they are indeed mistaken, and in that case require at least an Intel i5 machine of better still an M1 silicone model to be fully up to date. But beware, any M1 Silicon Mac if found defective outside warranty will be very expensive to repair as they are not truly modular/easily repairable as the earlier iMacs.