A lot of keyboards don't have two alts, but rather an 'alt' and an 'alt gr'. Lefties would be sad by the loss of the colorpicker vs the rotate widget.

Alt can't be used for the eyedropper as desktop environment commonly use alt+ click for moving windows.

I'm for an option to shift+right-click and drag for brush options. Either just adding a definable second variable to change like shift-clicking, or better (I think) would be a secondary radial menu, as mentioned before.
Even just right-click and drag wouldn't be too bad, as a click would just open the palette anyway.