My thoughts are this, whilst Instagram video has some interesting features that Vine does not, I like Vine because of it's simplicity. There are no filters, just immediate, raw video and that actually feels really contemporary and refreshing.
And I like the fact that Instagram is just images. I follow people on Instagram because I want to see their photographs. I go on Vine and follow people on Vine specifically to see their videos. Now I have to look at both in one feed? What if I love someones photos but their videos annoy the hell out of me? That would kinda put me off Instagram altogether.
It's not that I hate change, it's just I think Instagram is perfect the way it is and Vine is perfect the way it is. They're two very different things, and I don't think Instagram, or more importantly Facebook, understand that.
But that's just me.
Edit: Here's an example!
Vine is this:
Instagram video is this:
Wait... I can't even embed them. I'll give you an idea - it was someone doing a selfie for 5 seconds.