The people at Instagram aren't going to spend development time on the WP app unless they see a lot of people using it. Lets face it, to them the amount of WP usage is small and insignificant compared to the usage they see happening with iOS and Android. You can hardly blame them for not dedicating more time towards WP.
Facebook Beta gets updated very frequently. No reason to think Instagram wont get further updates as a result of several months in beta.
It makes more sense for developers to wait for 8.1 RTM than release something now.
WP market share growth is what will get attention. Would help if half the phone shops I've been to didn't dismiss WP as a joke, especially EE who's stores seem full of Samsung fanboys.
Your worried about the new update when they haven't even added video upload on windows phone yet. 6tag has video upload so I use that and donated to ditch ads. Rudy huyn outdid himself with that.