We are not forcing anyone to upgrade to Space, rather that Haiku is old code that is built using an old SDK. It is not trivial to simply issue an update as in order to do so we must update the entire app and UI for the new SDKs. That is why Haiku does not have the new iOS 7/8 look. At this time development has moved onto Space and we don't currently have plans to do such a major update to Haiku.
I will look into any options, but its not likely that Haiku will get updated. However, I know other apps are experiencing this missing slider as of iOS 8.3 so its very likely Apple will correct it in the next update.
All this being said, I do recommend users upgrade to Space simply because it is a much more polished application using the latest code and has the least amount of bugs. It performs much better than Haiku, I know this from personal experience in testing and real world use. We have lots of UI and feature updates planned and a big update is already queued with Apple. We're always happy to hear your feedback and it is our goal to make Space the best home automation app possible. Upgrading to Space also supports development of the product, which is very niche thus every sale helps. Sticking to Haiku simply limits us in what we can do for you in the future with Space.