One thing that would change my mind is if 90%+ of the Haskell community always switched it in, as you do. But the poll only showed 43%; substantial but not overwhelming.
Honestly, if we trust that ~45% of active Haskell programmers use BlockArgument, then I'd say it's a pretty convincing argument to include it. I'm absolutely not intent on using it (in fact, the mere existence of BlockArgument is a little baffling to me), but turning it on doesn't force me to use it. So if 50% of Haskellers care enough to actively turn on the extension, I think we can consider it a pretty compelling reason to have BlockArgument by default.