There is no rule that you can't create apps that "mimic the Twitter user experience". You have them confused with Apple. What there is is advice from Twitter that such apps are not a good bet, because Twitter is aggressively promoting their own apps and has no compunction about making changes that advantage their apps and disadvantage third party apps.

What other features might they create? Who knows? They're clearly going to bake in anything that makes sense for Twitter-the-business. That's a good thing, as far as I'm concerned.

I don't think you're right that Twitter is particularly dependent on third-party apps. I think we talk in a geek echo chamber that is particularly enthralled with apps, and with the best possible Twitter clients, but that most of Twitter's users --- and, in particular, Twitter's most lucrative users --- could give a flying whale about them. But they do need to share links and photos.