If you're looking to rent for the day, you would never pick 3 cars over 1. And if you already own the 3 cars, you wouldn't go out of your way to rent another one. I don't see how a parking charge would change this, unless it was far heavier than this proposal is.
Additionally, think about how many full 9-seat vans there are in Paris. Think about how many single-occupant SUVs there are. I think the benefit here is pretty clear.
Keymonk. It actually still mostly works if you find an old apk, I'm using it now. The suggestion bar is broken and it's a little buggy, but god most of the time it still flies, security risks be damned.
Keyboard 69 was another option, but it's also abandoned and I found it much more bloated and buggy.
What I wouldn't give for a good open source two finger swiping keyboard...