Intuit does not support software on unsupported OS versions, Apple is stopping support for Ventura this November, same is happening with Windows 10. 8 years is a good run for a machine, I went thru the same thing 2 years ago on a 2015 iMac with OS 11 so this is about the same lifespan. I kept mine running for a year and did taxes on an old MacBook running OS 12 as a stop-gap for a year (til OS 12 was dropped). Your iMac will survive for a while but obviously you'll stop getting updates from Apple and other third parties like Microsoft/Excel, Adobe etc will stop getting updates if you use anything like that, not just Intuit, your experience and security protection will start to degrade.
I replaced my iMac with an M4 Mac Mini and 2 external Dell monitors it's a much better setup for less cost and more flexibility than getting a new iMac.
Moving to PC entirely is a lot of work and a big shift, unless you're done with Apple and really want to move back to Windows 11 regardless of how you do your taxes. Just for Turbotax you need to carry over to a new machine the .tax2024 file to start your 2025 return. (If you did buy a new machine it would be Windows 11 which is fully supported - Windows 10 will not be supported by Intuit going forward).
