The reason there is no scriptural support for this, or any of the Marian dogmas, is because there is no scriptural support for them.
It is based on a lot of "it seems" and "it stands to reason".
Even the catholic encyclopedia online admits there is no scriptural support for the immaculate conception. None.
Ealgeone, I just looked up and read the online Catholic Encyclopedia entry about the Assumption, and it does *not*, as you claim, “admit” that there is “no scriptural support” for the Assumption. (You wrote “for the immaculate conception,” but I figure you must mean the Assumption because that’s what we were talking about.)
I’m surprised you would make such a claim, when any FReepers with five minutes can look it up themselves and see that no such “admission” is made.
It offers no survey of Scriptural types, images and themes, no theological or liturgical analysis, no description or discussion of the Dormition from a mystical or dogmatic Orthodox or Byzantine point of view, and, being published decades before Munificentissimus Deus, does not have a discussion of that either.
In short, you’re looking at the wrong article, ealgeone. This is not an in-depth study dealing with scripture or exegesis at all. It is basicly limited to a fast overview of the observance of the feast.