I agree with much of this, but I'm no clearer why a DT is better for a novice to learn on than a spey line. As you say, a spey line is basically just a long belly WF, albeit with a specially designed taper. However, I certainly didn't mean to suggest that a novice should start speycasting with a conventional WF - sorry if that wasn't clear.
I think a specialist spey line has two significant advantages over a DT. First, the front taper is typically much longer than you will find on a DT line, with the main weight of the line closer to the rod tip. It is therefore easier to transfer energy down the line and get good turnover of the fly when spey casting. Unlike with an overhead cast, the forward movement of the rod in the delivery stroke doesn't act directly on the tip of the line, so the transfer of energy from the belly is the only way the fly can be propelled forwards. Secondly, as with a conventional WF line used overhead, it is easier to shoot line into the cast because, assuming the back of the head is at the rod tip, the thin running line is so much lighter and offers less resistance than a DT.
I started speycasting with a DT line, because that's what I had with my existing setup; specialist spey lines weren't widely available at the time (mid 90s), though Michael Evans (who I believe was the first person to produce a modern spey line commercially) had in fact started selling them by then. But since these lines have become widely available, and in the past few years have got so much better, I don't see any reason to use a DT line, when a spey line will do the job so much better most of the time.
DTs are still useful on small rivers, where you may need to cast very different lengths of line and distance isn't a prime concern, but I think a spey line will still perform well enough in those conditions unless you're really casting a short line (in which case there is the option of using an overhead cast), and certainly they come into their own anywhere that you want to cast more than, say, 20 yards. This is true for novice or expert alike.