They accumulate gunk that was present in the cold water supply.

If its not quite hot enough you can end up having a nice incubator for bacteria

Yes, but water coming from the cold water tap has spent less time sitting in that accumulated stuff.

Seems like the presence of the accumulated gunk in the cylinder means the hot water must be cleaner by definition? The accumulation being the net difference between what was dissolved in the incoming water and what is dissolved in the outgoing water.

With the exception of gunk that is from the lining of the cylinder itself of course.

There can't be more minerals coming out than what went in though. It's not like calcium grows. Maybe it might break off and come out as some concentrated chunks, but overall there would have to be less coming out for it to accumulate in the tank.

Yeah, so the gunk has less time to fall out of the cold water.

Looking inside a kettle in London provides the same experience (at least when I lived there).

Now we live on rain water and the inside of the kettle is pristine, despite bird crap on our roof and flora in the guttering.

That's why modern kettles can be cleaned. I clean the mineral deposits out of mine once a year or so.

For things like an instapot where you add a cup of water to it, you are actually supposed to use distilled water. I used tap for years with mine and got all sorts of mineralization. It took quite a bit of vinegar to get that off, and now everything looks pristine since I have been using distilled water. You might consider using it for your next kettle, although perhaps the minerals in tap do something to the taste of the tea.

Do you buy hundreds of liters of distilled water? That doesnt sound practical