Are you theorizing that majority of content is posted through these third party apps? I can definitely see it's plausible that the content quality/quantity could drop. But if the average user is using the official app, will they notice or care? Doubt it. As for mods, I think there will always be people that will want to mod, using whatever way is available.

I'm pessimistic that it will matter, the power users will leave, the quality of content will drop, and the vast majority of users will be perfectly happy with the low quality content-slop that is left.