What you suggest is not sufficient. What would happen with existing vulnerable applications? You cannot remove VM_GROWSDOWN for them.
So we have some sort of mess. Reading the description, I think what got in is messier than Arcangeli's proposal. But I digress.
However, since VM_GROWSDOWN seems to be used just for the stack of the main thread (I think it wouldn't be possible to do otherwise for reasons already discussed), ditching it out would mostly make sense. _Except_ if you have a single-threaded application (and there are lots) which needs a big stack. So now each application has to decide whether to switch to the new layout.
For myself, I would enable the new layout by default, but I know somebody is going to complain; unfortunately, backward compatibility makes such changes hard.
Just my 2 cents.