When apps like Blender stopped working on my mid-2010 iMac is when I jumped.
My two 2015 vintage Macs are fine as browsers but wanted a fairly loaded Apple Silicon MacBook with a big SSD for various reasons.
Cybersecurity. You are 10x more likely to be hacked than someone running a modern OS with browser+os-level zero days patched. At least run Linux on it.
Why do you think they haven’t updated the OS and browser?
Also, how can you patch a zero day?
Apparently according to online sources, the latest OS supported on a 2010 iMac is macOS Mojave, and the latest security update for Mojave was released in 2021.