I gave it another try and it sort of works. But it’s very confusing and will lead to a lot of confusions. At least the way I understand it.
When I press the “1” shortcut (let’s just call is like that) and select an eraser after that, it’s remembered. However, when I select a brush after that, it’s used as an eraser and a pen icon appears next to the brush thumbnail. And then I can’t even turn the eraser switch off any more. It’s stuck.
So to be able to select a brush after I erased, I have to press the “2” shortcut first and then select a new brush. This is way too complicated and if you forget it, you’ve turned your brush into an eraser and can’t get it back to a brush.
The proper functionality should be that I can select erasers, pen and smudge tools whenever I want. And as soon as I press one of the three shortcuts, it should select the last used tool of the respective category.
About the eraser tool. There’s nothing special about that. It’s the eraser tool that pretty much every painting app has got. In Krita it’s a bit more confusing, as erasers are basically just a special sort of brush. So in Krita the eraser tool would be the subset of “eraser brushes” - the ones that by default are tagged with “Eraser”. The advantage of having a dedicated tool is that it would basically also serve as a “slot” like in the Three Slots addon. And before some users get annoyed again. I don’t want to replace the eraser switch as it is now, just add the eraser tool on top of that. And the “E” key can also stay assigned to the eraser switch. I can change the shortcuts anyway if I want to - and my Cintiq remote does only simulate the key that I tell it to.
But eventually I would also be happy about a proper Three Slots addon that works as described above. But if there was a dedicated eraser tool (and maybe even a smudge tool), user coming from other painting apps will immediately search for them and know how to use them. No Three Slots explanation required.
So if there is some skilled programmer out there - maybe the Three Slots addon could be adapted to work a bit better.