I have no option but to replace my aging Macs. Both desktop and MB Pro are stuck with OS 10.6.8 and are fairly limited in their usefulness now.
I'm wondering in what way you find them "limited in their usefulness."
I keep one Macintosh in my office running OS X 10.6, because it runs a couple of pieces of legacy software that didn't make it to be Intel-native, and hence require Rosetta, which was discontinued after OS X 10.6.
But the machine is still incredibly useful for most things. It won't run the latest software, but it still runs most of the software that it ever has perfectly and with good performance if you perform some routine maintenance:
Macintosh Routine Maintenance
OS X Maintenance And Troubleshooting
Safari under OS X 10.6 is now too buggy to rely on. While the only way to upgrade Safari would be to upgrade your OS, there are more modern versions of Firefox available (which are older than the most current version) that are both fully compatible with OS X 10.6, and which support HTML 5 and other modern technologies.
Firefox (48.0.2) (free)
<https://ftp.mozilla.org/pub/firefox/releases/48.0.2/>
is the last version of Firefox that runs under OS X 10.6. It's an excellent choice.
An old Mac might be slowing down or getting flaky because it's rotating disk hard drive is on its last legs. You can easily replace that hard drive, and drastically improve your old Mac's performance, by replacing it with a SSD. SSD's have recently come down quite a bit in price. e.g.:
SanDisk 1TB Ultra 3D NAND SATA III SSD - 2.5-inch Solid State Drive - SDSSDH3-1T00-G25
$140 with free shipping
https://www.amazon.com/dp/B071KGRXRG/?tag=macforums0e4-20
So, unless you badly need to run a version of some software that won't run under OS X 10.6, you can extend the life and performance of your old Mac pretty cheaply.