What you mentioned is a fun gimmick to draw a decent crowd once in a while, and stuff like that could help, but that's not how you sustain a league.
They need exposure. They need a twitter account people can find. If you type in "NPF" into the twitter search bar, the #1 result is "National Psoriasis Foundation" and the NPF twitter account doesn't even show up because "NPF" appears nowhere in their name or handle. Because nobody there has the first clue about how to actually run a league.
Imagine if, instead of @MLB, the Major League Baseball account was @probaseball. OK, even that doesn't really demonstrate how stupid the NPF is because at least the word "baseball" exists in my example. They don't use the word "softball," so if you search the most important word you won't see the league's twitter account pop up.
Of course, the account itself is worthless anyway. They didn't even do regular, live updates of their championship series in August.
I can't say for sure if a pro softball league could be viable, but I am certain that there are measures the NPF could take to improve their lot in life if they were at all competent.
EDIT: To be fair and somewhat kind, Bandits games are a fun experience, and that do a lot to make that game day experience a lot of fun. But it's like they're trying to keep this s*** a secret with how bad they are at informing people about the product.