Hi Jean-Francois,
Seems like I took some time to reply too 
, my apologies.
You make a great point here: the mechanics of how scrolling is triggered from a pre-saved size are not always clear. Although I feel we have two different issues here:
The first one is improving the mechanics and interaction to create scrolling artboards and the second one is how we currently handle viewing options.
I think there is a missing scenario, related to the second issue and pointed out by Steve in a comment in the previous post, where you want a size to fit the viewport’s width in smaller screens, but not in larger ones, to avoid the design from “stretching”
I agree that this shouldn’t be a new option, but rather a new behavior added to the existing Fit Width option.
About creating pre-saved templates we do want to improve how this works, and letting you create scrolling artboards more easily.
Thanks for sharing about how a Responsive option would work for you. This is also something we want to address, and I’ve added it as a new Feature Request.
I wanted to mention that the current Fit Width option could help in your use case, depending how you want your design to adapt.
For example, if your design uses a centered layout like here in the Sketch Forum, you’ll find that Fit Width will stretch it, that’s where a Responsive option would come in; but if you want your header’s elements to adapt to bigger widths, then it Fit Width will do the job.