There’s a good deal of misinformation here. The main part being disk space. While it is true that flatpak apps will take up more space, it’s not nearly as bad as you think it is. There is a lot of really good optimization going on under the hood that you don’t see. Dependencies are de duplicated. I’m no expert on it, but I believe that dependencies also have delta changes from one version to the next.

Regarding apps not supporting building of the source, you should get over that or do the work of supporting it yourself. Open source is a hard, usually thankless job.