It would be so very nice if this were to be false by default.
counting 103 in my setup, I wonder if I am alone hoping this would one day be reversed
This is totally a hold-over from the pre-Lovelace days when cards were based on group membership. The header kind of made more sense here because it would toggle the group. When you can have arbitrary entities in a card, some of which don’t even respond to on/off commands, it makes a lot less sense. I have it turned off on all of my cards as well.
As reference, the current logic is: As soon as there are at least two toggle-able entities in the card, the show_header_toggle defaults to true. For other entites it is not set, so I think it is kind of reasonable.
This logic was added with this PR https://github.com/home-assistant/frontend/pull/4964.