In terms of cartridges, I agree that it would /should be / is easy to change over. Is was more meaning the cost of keeping multiple different types of cartridges on hand. Currently ink is the most expensive part of a home printer, and I can only see that increasing as one needs to have a cartridge each of many types of plastics, rubbers, ceramics and metals on hand to print out anything. A dedicated print shop seems like it would be better able to stock multiple materials than a home printer would. Perhaps dedicated photo printing shops might not exist any more, but (around here at least) they do feature in most office supplies, electronics and department stores.
As for the cost of photos currently I can print photos at 10c per photo plus ink. Or, I can get it done at a office store / print shop for 10c per sheet, no ink or hardware costs... I don't think any significant amount of people buy packs of over 100 sheets of paper at a time? A quick search for suppliers didn't find me anywhere local that sold sheets for significantly less than 10c per sheet.
The other argument I see against 3D printers is the inability for a large number of people to create decent content themselves. This may be a skill learnt over time, and the market in downloadable meshes would probably explode if this did come to fruition, but I think it is unavoidable that realising a decent 3D model for printing is significantly more difficult than a 2D print.
Are there really things that you would use this for every week outside of Wargaming? I must admit, I don't find myself missing bottle tops ever, and the only times I'm short enough on cutlery to need some printed it would probably be cheaper just to wash the stuff I already have

I've used shapeways for wargaming, and I can definitely see the use of 3D printers in spare parts for various things, but on a weekly or even monthly basis, not so much.