Being a maple syrup producer I use loads of cordless tools. I replace the batteries when they'll no longer take or hold a charge. Most are DeWalt, but I have 3-4 other brands. I think I get more for my buck buying aftermarket batteries in the largest AH size offered.
My wife has done well buying yard sale tools for me too. Once she bought a whole set of tools, Ryobe (2 18V drills, 2 18V circular saws, a cordless light, a cordless miter saw and a cordless reciprocating saw). I bought a charger and 2 high AH replacement batteries. I've used them plenty over the past 12-14 years. Recently she picked up a 14.4V drill, impact 1/4" driver, a carry bag, 2 chargers , one good battery and 3-4 no good batteries. All DeWalt, since I had all 18V and 20V DeWalt, I ordered 2 high AH batteries, aftermarket, both tools work fine. On 18V DeWalt batteries I used to buy the twin pack DeWalt batteries from Lowes when they put them on sale. I've pd as little as $79 less 5% for 2, normally $89 for one.
The only time I get rid of a cordless tool is when it no longer works with a good battery in it.
I likely have 25+ cordless tools and they all get used regularly. The main thing is to keep the NiCad batteries charged, draw them down to zero then recharge, on the few lithiums I have, charge them anytime and keep them full.