I honestly don't 'get' the terrorific nights - I mean, it's Disney, not Hammer House of Horrors . Not suitable at all for children.
They sell out though where the MNSSHP didn't, you could buy tickets on the day, and the TN are full of parents bringing their tweens and teens as well as adults who might never have come to DLP before and that might come back after to the park.
Halloween isn't popular in France, it was unknown before the late 90's then after immense commercial bombarding was adopted for a few years and has now pretty much died a death.
Despite commercial efforts (!), some people don't go mad about Halloween in the UK either - not like the USA anyway. It is steadily growing in popularity however.
I see your point about the Terrorific nights selling out but honestly, could they not even hold one or two MNSSHP nights : They didn't need to run as many and then perhaps they would find them full...?
I honestly wonder what Walt would have thought about the Terrorific nights - having seen the videos, I really don't see how it fits in with the Disney ethos... But I suppose holding them in the Studios fits better than the MK....
We celebrated Halloween when I was a child back in the 70's as did everybody I know/knew so to me it's always been around. Even my parents did apple bobbing in the 50's ...OH had no idea what it was all about until 1997 which was the first year here that we got it
The MNSSHP only started in 2008 by which time all the fuss about Halloween was over here...they should have started them earlier in the 00's
As for what would Walt think or say it's impossible to know and leads to many a debate over on the US boards - The BOG controversy comes to mind here