Which is a great argument for going with an established brand. Makita or Dewalt will still sell you a NiCd battery for a 30 year old tool. My oldest makita lithium tools were bought in 2005 when they first came out. It came with three batteries. One died in about year 15, another one just a year ago, and one still works, although it's about half capacity. I have had one newer battery fall just outside of the warranty period, but other than that I've been amazed by their longevity, especially compared to the old NiCds that would only last for a couple years.
I would expect stihl, ego, ryobi, makita, Dewalt, and Milwaukee to be around and selling batteries for as long as any of us are.