You won't be forced to use Hangouts. This article I think casts a little light: KitKat SMS Situation Revisited: New SMS Rules Ensure There Is One App In Charge Of Your Messaging History, But Everything Else Is OK
If I've read it right, KitKat's changes include setting one messaging app as "default", message handler, but any qualified app can request to be default. So there's nothing to stop you making the HTC message app the default if you prefer (or Hancent, Chomp etc I assume, once all of these are updated for 4.4). I'd expect other apps can view messages, and that you can send from other apps. So while Google may just bundle Hangouts as a single messaging solution, there's nothing to stop HTC keeping a separate SMS/MMS app (whether Google will provide the APIs to let you use Hangouts messaging from another app I don't know).