Interesting, I also use a T460p (32 GB with a second m.2 harddrive), my battery still has 92 % capacity after more than three years. Do you also have a Sanyo battery? I think batteries are often sourced from different companies so you might have gotten a bad one. Did you often completely discharge it? I heard that charge/discharge patterns can have a large influence on the capacity.

I also really like the T460p, three years in it still works great, is easy to upgrade and not too heavy.