I feel the same way.
Every year, my family attends at least four conventions: BronyCon, Ponycon, Dragon Con, and New York Comic Con. This has been regular for us since late 2012, and I attended my first fan convention (AnimeExpo) in 1995. I love lots and lots of geeky things, so I greatly enjoy all of these events. But of them all, BronyCon is my favorite.
I do attend for the camaraderie - the brony fandom is the nicest, friendliest, most welcoming one I've encountered - but as a strong introvert, I mostly come for the con events themselves. The panels, the dealer room, the concerts - all the things that are just not the same when you're not experiencing them in person. As others have said here, I worry that the quality or quantity of those will suffer. The convention only has so many resources - time, money, space - and I don't think there's a shortage of MLP content to commit them to. That means anything else will necessarily detract. They already can't accept every MLP panel application, can't invite every MLP show staffer, so I don't think it can be argued that giving those resources to something else won't take away at least a little that could have gone to MLP.
The argument, especially in light of last year's attendance numbers, might be that bringing in more varied content will bring in more attendees, which will bring in more money. Depending on how resources are allocated, it could mean more MLP content than they'd otherwise have. But as many cons as I've attended, I've never been an organizer, so I don't know how that might work. (I also find it hard to believe that such a severe attendance drop-off had anything to do with anything but people having other July 4th plans.) In any event, I'm not looking for another Dragon Con or Comic Con. When I've considered travelling even farther from my east coast home for additional conventions, it's only been for other MLP conventions.
BronyCon is also under new leadership this year, and I think a lot of us don't know yet what that means, and won't really know what it means until we see for ourselves in August. The former con chair has expressed support for this move on Twitter with a statement to the effect of "change or die" (I'm paraphrasing) but the question is, what kind of change? Is this the best kind of change we could have? How true is that statement, anyway, and are we just changing for the sake of change?
In addition to being a brony, I've been a trekkie since TNG premiered when I was a kid. I look to the Star Trek fandom, which is extremely healthy in spite of no new TV show in 12 years, only some AU movies which most die-hard fans don't care for. Similarly, MLP is an enduring franchise. FIM will eventually end (I assume; I don't know that we want it to go on forever with a decline in quality like The Simpsons or South Park) but MLP will not. This is, after all, Gen 4, and whatever we each think of the earlier generations, I think Hasbro has learned a lot from FIM's truly massive success, so I have high hopes for the inevitable G5 and beyond.
What all that means is that the MLP fandom will still have new "official" content to sustain it. (I'm setting aside the truly staggering amount of high-quality fan content only because none of it is as universally seen, and thus as unifying for the fans, as the show itself.) I was also a huge xenite back in the day, and while it still warms my heart to see a Xena, Gabrielle, or even occasionally Joxer cosplay, I recognize that an annual full-fledged Xena-only convention would not be feasible in 2017, so many years since the show ended. But I don't think that MLP will go that way, so I don't think that by itself is an argument that BronyCon 'needs' to incorporate other things to survive.
I don't want to say that the sky is falling, though. It does seem like the current BronyCon leadership is doing this cautiously and incrementally, though I don't know what their success metric for determining whether to continue will be. I'm also struck by the fact that so many comments here express concern, while they keep retweeting those who express support. I fear that those of us with concerns are being lumped into a bucket as wide-eyed crazies who fear any and all change, or at least that there's some confirmation bias going on.
We'll see. I already have our silver sponsor badges for this year, and I'm not going to cancel my plans based on this announcement. I doubt most people will. So there's a sense in which all any of us can do - in spite of all these comments that I hope they read - is wait and see. I hope I'm wrong and it's as fun as always, or at least that if it isn't, they're willing to see that, too.