I believe you on the technical details, but as an anecdote, I have M12 batteries that are 10 years old and still working fine. At least, good enough that I have noticed no issues with them and I don't even know off the top of my head which of my batteries are newer and which are older. I also have a bunch of M18 tools and batteries, and I've noticed no particular difference in how they age compared to the M12 ones. But I'm just a DIY homeowner, so my usage is relatively light.