I can attest that my 9 year old pack is doing all I expect it would at this point. Being a 24 Kwh pack, it was limited from the get go.
As to "More than the car is worth" that can be looked at many ways. Try finding an 8 or 9 year old car with mileage below 100k, and compare price to what the Leaf brings in the same mileage/age. As to whether it is "worth" doing, that is a personal choice. You can buy another used Leaf and go though the cycle again, you can buy another type of car and move on, or you can invest in what you have now and continue on with a car that at least one point, drew your interest.
No one answer will fit everyone.
As far as Nissan installing a used module, I wouldn't expect they would. But in the aftermarket, there are opportunities, both in the used and new market.
When I was in trucking, I did so with 30-40 year old tractors, Many told me I should move on to "newer and better" tech. Yes, the mileage was slightly better on the new stuff, but I could repair the old for a small fraction of the cost of repairing a newer one. Aftermarket parts can be as good as original or junk, and in some cases better than original as newer tech gets incorporated into the older parts.
No reason a older Leaf couldn't get a new lease on life and turn another "lifetime" with a battery replacement and can even be upgraded with larger capacity so to be more useful than it was when new.
I could overhaul a large Cummins diesel for about $1500 and some work. I would then have a "new" engine in an old truck that would than run another million plus miles. This isn't hypothetical, I ran them that way for close to 30 yeas.