I always lean towards keeping the car if it's regular wear items like suspension, CV boots, etc. You know the service history, and fixing stuff like that a time or two a year is a lot cheaper than a new car payment, or even a used car that you don't know the service history on, especially if you're doing the work yourself. Once most of those repairs are done, you shouldn't have to worry about it again for a long time. As you get to know a car, you get better and faster at working on it too. Engine and transmission problems are usually the big ones when it's time to throw in the towel in my opinion, unless you're a true enthusiast for the car...
The Mazda axles seem to be pretty robust - why not just replace the torn boot? I know mechanics don't like doing this, mainly because the time involved makes the final cost to the customer the same, but I think for a DIYer it makes a lot of sense. Is it the inner or outer that's torn?
I've done the repair twice on my driver's side axle. First time, a little after 100k miles the outer boot tore and I put on Empi brand boots on both the outer and inner. They fit fine, but the rubber will break down a lot faster on them than the OEM boots. About 40k miles and 3 years later I noticed they were starting to get pretty big cracks so I decided to replace them with OEM boots before they failed. The OEM boots were heavily discounted on Amazon at the time and only about $20/piece. Not sure if you'd be able to find them that cheap now. It's a messy job, but not too bad.
I did it by removing the tie rod end and lower ball joint (the worst part of the job in my opinion - use Kroil oil to help loosen it), and removing the axle nut. With all that off, you can then remove the clamps on the inner joint and take the axle out without removing the part that goes into the transmission, so no need to drain the transmission fluid. To finish the job, you'll need snap ring pliers, CV boot pliers for aftermarket boots, or for the OEM Mazda boot which use a different kind of clamp on the inner boot, I found a set of really long 11 inch 90 degree needle nose pliers were needed to snap the clamp in. If you look at the clamp, you'll see Mazda didn't use a standard CV boot clamp on the inner boot for some reason, but on the outer boot, did use the regular CV boot clamps.