Graffiti is an odd thing. If I wrote in nicely formed letters TOM 2021 somewhere, people would look in disgust and it'd make me an ***h*le.
But if I'd done the exactly same thing back in in 1821, you'd admire the work
Spot on buddy the amount of times we have said and thought the same thing, when its new " nothing but vandals " when its old its admired !!
I think there is almost an agreed convention of 50 years old becoming worth preserving?
I think it's about context. Graffiti done when a mine was working is obviously of historical interest and one can't really decry the graffiti of those that dug the mine in the first place! I imagine at the time no-one cared much.
Graffiti of historical visitors and explorers in previous centuries/decades and reflecting post-mining uses is also of interest as it adds to the history of the site.
Even some modern explorers will admit to adding a name and date to the far end of difficult to access/remote areas. Similarly this perhaps also adds interest and acts as some sort of record.
I think to add such things to a site you really have to ask yourself carefully what your motivation is and if you are adding anything positive by doing so - not to mention what you are potentially covering up!
Some fellow in the 1800's exploring with a candle was probably doing something pretty exceptional at the time. Someone wandering through Wapping these days with a can of special brew, LED torch and some aerosol paint probably less worthy of permanent record...