iMac (21.5-inch, Late 2015)

I agree with David's comments but can offer a slightly different perspective.

I too have this late 2015 iMac. My wife was using it, but it was very slow due to its original internal HDD (mechanical drive). We purchased a brand new M1 iMac for her and she is thrilled with it. Meanwhile I decided to see what I could do with her old 2015 iMac. I replaced the internal HDD with a 1 TB SSD, a total cost of about $200 including the new Samsung SSD plus labor from an Apple Authorized Service Provider (I would not try this myself on an iMac). Even though this computer has only 8 GB memory, it is like a new computer now. Very fast. I am using it myself now.

I have some experience with old Macs, as in addition to our new ones, I have a 2010 MacBook Air (all original equipment) and a 2013 MacBook Air (which has a new internal SSD and new battery). The 2010 one is limited to High Sierra, the 2013 one is limited to Big Sur. We used to also have a 2008 iMac but just recently recycled it as its power supply finally failed.

Here are what to watch for with an older Mac:

• Some emails don't show text or graphics content. Due to older MacOS and old Apple Mail version. Can be addressed by viewing such emails in the web browser interface.
• Some web sites don't load properly in Safari. Can be addressed by using Firefox or Chrome for those.
• Some latest versions of some software won't run due to older MacOS. For something like tax software, this can be a showstopper, and is why I got a new MacBook Pro recently as I have to be able to run the latest tax software for personal and business use.

I see none of the above problems with the 2015 iMac which is still running the most recent MacOS from Apple. Since it won't be eligible for Ventura, some of these symptoms will likely start to appear over the next 2-4 years. But waiting as long as possible to get a new Mac is an advantage because as time goes by, each new Mac has new features that the previous one did not have.

As for parts and repairs, work through a local Apple Authorized Service Provider. They have the right skills and know where to get suitable parts. Apple itself stops stocking parts for or doing repairs on vintage Macs, but third party suppliers do exist and the AASP can tell you which ones are best and the AASP will still do work on older Macs. This is how I revived the 2013 MacBook Air, the SSD and battery replacements were from OWC, very inexpensive to replace them. That computer is still fast and responsive.

Keep in mind that older hardware can and does fail eventually. But Apple hardware can last a long time, and some of these older Macs have fast processors (like the iMac 2015) that continue to work well with modern software. The internal SSD seems more than adequate to compensate for having just 8 GB memory. My 2010 MacBook Air has only 4 GB memory and even it does fine, even with Adobe Lightroom, MS-Office, etc.

This mac is now classed as Vintage. The late 2015 iMac (which I also have) is not capable of upgrading to macOS 13 Ventura when it is released later this year; this iMac can't be upgraded beyond Monterey. There may be the occasional security update, but the 2015 iMac has reached the end of the road.

Additionally, at seven years of age it's done well and will continue to do well with Monterey for some time if you had hopes of it running a bit longer.

In terms of any future repair, this model won't be worth repairing if a key component fails. The money would be better spent towards a new computer. There is also the question of availability of parts, but only an Apple Authorised repair technician could tell you that when you present your Mac for inspection.

Your question hopes for some good news about greater longevity of your Mac. Sad to say, it might be time to activate your hardware upgrade plan.

The latest macOS 12 Monterey version should be supported by Security Updates for at least 2 or 3 more years.