Replacement batteries are fairly easy to be had. Replacement of RAM depends on the model, but for a 2010 model, for instance, all you have to do is remove some Phillips screws and crack the case: https://www.ifixit.com/Guide/MacBook+Pro+15-Inch+Unibody+Mid+2010+RAM+Replacement/3032
Yosemite is definitely a bit slower on older models, so unless an app you want is Yosemite-only I think it's very much a personal decision. Things like Handoff, etc. require newer hardware than your computer anyhow.
As you say, all this is irrelevant if you decide to get a new model. I'd say you wouldn't see much raw performance increases, but they do have generally longer-lasting batteries and are much thinner and lighter, so those might be worth just getting a new computer at this point. Your 2010 is probably worth a surprising amount of change via something like eBay.
I'll definitely look into the RAM/battery part then. Like even the Macbook, looks awesome but man, the performance isn't great and neither is USB C. If only the MBA had a retina screen.
Was thinking of using local classifieds in Canada (Kijiji) but what would you say for a well-used, battery not so great, 2010 Macbook 15" Pro with 4 gb of ram, 2.66 ghz?