With Musk's established track record I think it's pretty arrogant for anyone to call his ideas foolish. He has achieved things that people thought were impossible or bound to fail. You can say: “I can see this potential problem”, or “I wonder how he's going to deal with this physical limitation” or similar sorts of statements. That's feedback and analysis given with respect. Simple dismissive negativity against ideas put forward by a person who would already rank in the top 10 in the world today for technological and engineering achievement is arrogance and narrow-mindedness.
I dislike this sort of miscasting of what I just said. I'm a very strong proponent of free speech and the right of anyone to say anything. But with my support for free speech comes a commitment to try to be considered and careful in what I personally say and I'm encouraging others to do the same in this particular context. You can take it or leave it and of course you can go on calling him foolish if you like. Musk doesn't probably care a whole lot and in the end, I would say that the negativity simply reflects back on you.