It's impossible to predict where the show would have gone without bronies, aside from the fact that it would have likely ended at S3E13. Without the energy that bronies bought, would the staff have felt the need to keep pushing themselves in S2 and S3? Outside of the US, the franchise probably wouldn't look very healthy, as Hasbro did a really bad job with international marketing (admittedly, they might find it would have been easier to sell the show to international networks). Hasbro would also be missing WLF merchandise revenue, and the comics would have never progressed past the clip-art ones in magazines.
Return of Harmony was in the S1 production block. Discord predates bronies, and I feel like it's likely they might have still brought him back if bronies never happened.
It is bad that kids can't search for favourite characters without horrible things being suggested in the search suggestions. It is bad that some bronies are abusive, abrasive arseholes that never seem to learn anything from the show they claim to love. It is bad that others exhibit the geek social fallacy of ostracizing being evil, and thus siding with the arseholes rather than the people they're attacking. But these people would just be doing the same thing about something else. The problem is not fandom specific, it's just a lot easier to see a fandom's problems while inside it.
I do genuinely feel that the show has inspired some to become better people - connecting with others, getting over some hang ups, maybe getting a reminder on a lesson that didn't quite sink in the first time round.

It has inspired a lot of people to become more creative. And I don't know how many people would have ever had this inspiration without connecting with others, how they'd have even started watching in the first place. The bad is still really bad - the good doesn't counteract that - but it's not as simple as "all bronies and everything they ever did are scum". Fight against those who wish to harm others. It doesn't mean you can't appreciate the good.
