I never had problems with devices becoming slow where a factory reset does not help, this sounds odd since a factory reset clears all configuration data. I tend to install some AOSP-derived distribution on devices which are supported (or which I port it to myself), for those for which I can not find a working distribution I pare down the factory distribution as much as I can by removing bloat and replacing obnoxious apps with functional ones from F-Droid. This makes it possible to use e.g. a Samsung Galaxy SIII from 2014 with Android 11 as a daily device for 'dangerous' work.

The problem is not with the data but what's under it: a flash memory chip the manufacturer cheaped out on.

Never had problems with those either, who made those Android devices with such short-lived eMMC chips? I'm using a number of old Android devices from both well-known manufacturers like Motorola, Samsung and Sony as well as lesser-known ones like Ainol, none of these suffer from memory-induced slowdowns. In my experience it is the battery and the touch screen which are most likely to give out first. The battery can be replaced, sometimes the touch screen can as well but I tend to just use those devices for purposes where I don't need a screen (which I often actually just disconnect to save power) such as MPD-based media players, a trailer camera and IP cameras.

I gave my Galaxy Nexus to my dad, so it saw longer use than usual. After five years it was virtually unusable. It was already slow when I stopped using it.

My Nexus 5, which is sitting in a drawer nowadays, also becomes ever slower. Even my HP Veer, keeping the Nexus 5 company, isn’t getting any faster either—even though it never saw real use.

So yeah, basically all manufacturers “do it”. It’s not even about heavy use or anything.

>who made those Android devices with such short-lived eMMC chips?

Asus/Google did, the Nexus 7 would brick itself after a 2-4 years from failing/worn out eMMC flash.

A guess: You don't (re)write data on your phone storage much? This experience is common for my girlfriend who takes 50 GB of photos every few months, transfers them all to the PC and does it again, but uncommon for me who practically never uses the internal storage except for apps.

Not really, these devices all have seen intensive use and are still being used in that way. Some only have a tiny amount of flash on board (e.g. Motorola Defy only has 2GB, I have 5 of these running as media player, MPD player, trailer camera and IP cameras) yet they persevere. Some (e.g. Defy) even have been used with swap-to-eMMC for a while, I ended up disabling this because it is just too slow but it made it possible to run some apps which otherwise would not run on 512MB devices. In short, I have not had problems with flash wear - yet.

Just because it is running Android 11 does not mean it has seen meaningful security fixes in the last half decade.

This x100!

I have an S8 and although I am a Dev, I am really a Luddite on my phone. I have some random apps, a Google account and things like WhatsApp and Telegram but otherwise not really anything of note.

However, I find things like the share photo button sometimes taking 5 seconds for the list of who to share with to come up. How? Sometimes I click an app and it sits there for as much as 10 seconds before something comes up, tbf, that might be the app but I fail to see how compiled apps can become slower and slower over time by this much.

I suspect the enemy at the gate is tracking/metrics type apps that are sending a constant stream of drissle to the internet to try and get "actionable data" for their Lords and Masters but just make everything appear crap. I also loathe the need to fiddle all the time. Google Authenticator, absolutely fine until they decide that we need to press something to reveal the code, now it's 2 clicks instead of 1 50% reduction in productivity! I think they have reverted it but honestly, leave things alone!

Sometimes it's Google services. I've had tremendous success with LineageOS (and CyanogenMod) over the years.

a prominent example of this was the first google nexus 7 tablet, née Asus Memo Pad. Therein somehow the NAND flash became used up in some strange fashion making write speed unbearable. This also could not be fixed with a factory reset.