You're right, flash degradation and deterioration of write speeds is pretty much primary reason why older phones feel slow and laggy.
A lot of - especially older or mid/low range - phones have cheap eMMC storage which is signifcantly worse at wear leveling than the higher end UFS storage.
> phones have cheap eMMC storage which is signifcantly worse at wear leveling than the higher end UFS storage.
Which is shocking really - the phones should switch the eMMC to RAW flash mode (ie. no wear levelling), and then write an actually-smart wear levelling algorithm that runs in the OS.
The OS has far better info for wear levelling anyway - it has more visibility into read-write patterns, it has more RAM to store more state, it can cron background scrubs and reorganisation to idle periods, it can be patched if a bug is found which only manifests after years, etc.
Unfortunately, as far as I'm aware, most eMMC's can't be put into any kind of RAW mode anyway.
A lot of - especially older or mid/low range - phones have cheap eMMC storage which is signifcantly worse at wear leveling than the higher end UFS storage.