I agree with Robert, not worth the effort (and may be against esri usage agreements since you are basically trying to reverse-engineer the software). I also don't like the dependency, but at least with Portal you can keep it more in-house.
The option you have is to write your own using ArcGIS API for JavaScript | ArcGIS for Developers There are samples so you can write many of the functions that WAB has, but it will take a lot more work to get it all to work. Up side, you don't have to have as heavy of an app for those things you don't need and you can make it much easier to use your own services (without dependency on AGOL/Portal). Downside, not as fast to whip up a new site.