The damage your seeing is as d8mok says very common on old parts.
Basically the process of anodising doesn't create a constant solid layer of modified aluminium....... its was explained to me in laymans terms as a cell structure with a solid base layer, like honeycomb. The solid base actually protects the aluminium, whilst the comb acts as carrier to colours and is part of the structural change to the aluminium the process causes.
Problem is the cells attract dirt ( dulling) and chemicals from cleaning, polishing, outdoor use, salt...(let alone physical damage).....you name it! These get into the structure, over time rot the base layer, then get under it, spreading out and corroding the alluminium.....that's what your seeing. Corroded aluminium under the modified anodised surface.
Thats why surface polishes won't do anything...your still polishing the top....you need to polish under it.
So when they get like that its either...
live with it
polish it right through the anodising back un modified aluminium
re anodise
Paint
If you leave it long enough you often get whole sheets of material that will lift away. We saw this a lot on old motorbikes engine casings that live in much harsher conditions. The anodising layer is still kinda there, but lifting off the damaged alluminium under it.
Its rare for bike parts to get that bad, but i did find a set of xtr cranks that i literally used a paint scraper on to remove the top layer
