My theory is there's a dip then a lift.
Many people agree 5 books is a good length and interest dips at book 6. I found my launches after book 6 were smaller. The rank didn't hit as high. However, it was only after 6 books that I noticed new releases had a positive effect on earlier books. Not enough to make up for the loss of the earlier books getting old, being pushed down by algorithms, and having exhausted their niche audience, but enough that I could see how carrying on to book 20 might not be so crazy.
My fans might not like this other theory I have, but here goes: trad-pub might be onto something with their 1 book per year (per series) release schedule. If someone binges through your books in a week, those characters aren't going to set up permanent residence in their heads the way they would if the books were meted out one per year. The brain doesn't retain crammed information the way it does information that's repeated and reviewed over time.