I just think that, as you implied, it's a harder "sell" than something like Batman or Iron Man.
I think this here is an example of how quick success can alter the perception of a characters mass appeal. I recall in the months leading up to
Iron Man it was a foregone conclusion it would
NOT work. Due to a multitude of alleged factors.
Not saying you
Spiff specifically but the 'net had a large share of posters who felt
IM was doomed to do at best $125m domestic.
Now the exact same compaints are hurled at Thor, GL and to a lesser extent Captain America.
Thor due to its a)mix of genres b)lack of knowledge on Marvel's Thor and a few others.
GL due to its first trailer with questionable CGI(which IM also had but we know how that was tuned up) and out of character humor.
Cap with his 'silly' costume and being a period piece.
At least one if not two of these will break out I think. So that in 4 years we may be saying,
" "x" is a harder "sell" than something like Batman, Iron Man or Thor."