Thanks for bringing up this point. I have felt for some time now that single app bundles are the way to go. If Apple can do it, then all developers who write apps for the Mac platform should be able to do it as well. In fact, I think that Apple should lay down the law, and demand it of all macOS developers.
Furthermore, I think that Apple should eventually do away with the need for an app to have a folder in the ~Library/Application Support folder, in the ~Library/Preferences folder, and everywhere else where current apps to need to place files in order to function properly on macOS.
Everything -- and I mean every single file that is related to a particular app -- should be located in the app bundle in the "Applications" folder, and nowhere else. If it were eventually possible to do this, just think what a huge step it would be for our computer security. If an app takes even one step to install something anywhere else, it would immediately raise a red flag and send out Apple's search and destroy bots.
The above may sound a bit extremist to you, but if we are going to talk about sandboxing, then let's REALLY sandbox all the way.
I have long wanted this!