The spinrite[0] user group has noticed some of these effects, even on in-service drives.

The theory is that operating system files, which rarely change, are written and almost never re-written. So the charges begin to decay over time and while they might not be unreadable, reads for these blocks require additional error correction, which reduces performance.

There have been a significant number of (anecdotal) reports that a full rewrite of the drive, which does put wear on the cells, greatly increases the overall performance. I haven't personally experienced this yet, but I do think a "every other year" refresh of data on SSDs makes sense.

[0] https://www.grc.com/sr/spinrite.htm

If full rewrite helps doesn't it sound like TRIM implementation in SSDs is buggy or insufficient? Or internal cell wear-maps aren't detailed enough. Anyway plenty ways it can go wrong, SSD firmware had also plenty of high profile bugs, including total bricking.

There are lots of other potential causes for the same effect...

Eg. Data structures used for page mapping getting fragmented and therefore to access a single page written a long time ago requires checking hundreds of versions of mapping tables.