iMessage is a lock-in for me mainly because I love the features it provides me and the connectivity to my family. It is simpler than getting everyone set up on WhatsApp or Skype for example (Skype is terrible for battery life on the iPhone as well) and just works.
You can however as you stated turn off iMessage on the iPhone entirely, at which point the Apple iMessage server will simply tell your friends you are not available on iMessage and send it over standard text.