"Some applications now are written as frontends to APIs and the application does not provide any rendering on the server side besides a nice error message that the website requires JavaScript"
1. Fix it
2. Stop using those APIs if they will not fix it (otherwise things will not be fixed)
"Many modern web applications can be much more performant because they take advantage of JavaScript"
That's nice, but I do not want you offloading your computation onto my computer. I may allow something like Folding@Home, but why should I allow something like rendering advertising?
"User interfaces that depend on JavaScript have much better abilities to make it enjoyable for a user"
User interfaces that work with or without Javascript and only use Javascript as an enhancement give you the best of all worlds. It is not impossible, you are just being lazy and trying to shut out people who disagree with you.
"A good example for instance is payment handling on the internet."
This is kind of like saying, "Every car should have OnStar; a good example is refueling!"
"You don't get extra privacy by disabling JavaScript"
Javascript on its own is sufficient to carry out numerous privacy violations, and presents various security problems. Yes, there are other things you can do, but if I stopped you from using everything else you could still use Javascript to accomplish your aims. Javascript also presents problems for Tor users, who might be de-anonymized (accidentally or deliberately) if they allow Javascript to run.
"At the same time I can enhance your browser experience"
Please do not do this; I do not want you "enhancing" my browsing "experience." All I want is to get the information I asked for, maybe post some comments, and manage my finances. I do not need any of these websites to have fancy UI effects, and while client side validation may be convenient for me it is not something I will miss terribly. My browser already knows what tabs are, how to load images, and how to submit forms. What are you adding that is worth the added risk of Javascript?
"We as developers should be happy that browsers go our way and make our life easier."
Sorry but my browser is supposed to work for me and not for you. If this is your argument for Javascript -- it makes your job easier -- then I see no reason not to keep it disabled by default. If life without Javascript is so terribly hard, the problem is with your techniques, frameworks, and designs -- fix those and leave my browser alone.