Any dumber and they'll float away

You're being very generous, Squonk. 😉

It was a guestimate

They are retarded, no doubt, but their heads would need to be filled with a gas lighter than air in order to float.

Please READ !
It says "any dumber".
That literally MEANS dumber than they are now !
So, at their present dumbness, they are STILL ambulatory here, Comrade.

Read the title.

I'm glad you did. 😎