Leaks and Compressor are the two primary reasons to replace an A/C. I don't know which is the more common, but I can say that although compressors DO fail, it isn't that commonplace. Do a poll; you'll find rooftop A/C's 10+, even 20+, years old. They've hopefully been cleaned, maybe had a fan motor or capacitor replaced, but still running.
If a Capacitor was $200, I'd say scrap the A/C now. But when you can get one for $15-30, or a meter to test yours for about the same price, I really wish you'd look at the Run Capacitor.
Said it before, will try again: A Start ASSIST (Hard Start Kit) is an Assist, it will not overcome a completely failed RUN capacitor.