And before that what was Houdini doing with the
alt key?

I hope still nothing, because if it was used for something and it was ditched for Maya style navigation, that would be a hard kick in the something to interaction easiness and versatility.
Again, there's a Maya style interaction in Softimage too (created when the 3d gods decided to throw it in Autodesk's hands) which allows you to navigate with
alt and adds a lot of other Maya hotkeys all over the keyboard, remapping the ‘S’ key as well (the default nav. key in XSI) and you get no overlapping.
Of course, the default Softimage keymap (the one that uses
alt as a modifier and ‘S’ for navigation) is much better than the Maya one and by opting to navigate with
alt you lose the advantages that comes with that.
Let's have a keymap profile for Houdini users, with
space for navigation and
alt for various things (imagine having three modifier keys, not just two) and all other familiar hotkeys and a Maya profile with
alt for navigation, space for maximizing/minimizing viewports, F8-F10 for point, edges, etc. and other hotkeys they're used to.
That would be the best for everyone, making the user solely responsible for their decisions, like whether to go with the overall well-thought Houdini style or with the silly Maya one.