Agree. And I had the same thing in mind. However, after some searching and going through the literature on my own I've come to the conclusion that a Diesel specific oil is always better than a full synthetic that is mainly gasoline SN rated with Diesel CF and European B3/B4. Take for example this Delo 400, it is CJ-4 rated and has ACEA E7 E9 certifications which supersede the obsolete CF rating the expensive Mobil 1 offers and the E7 rating is for heavy-duty diesel which easily supersedes the B3/B4 rated oil. High TBN does provide better alkalinity coverage against soot but an HDEO like Delo has other additives that help against up to 2000ppm sulfur and other deposits considering our environment.
So IMO Delo 400 is a nice balance b/w price and quality, and expensive gasoline full synthetics with SN/CF are better suited for petrol cars or places where they have ~50ppm or lower diesel, or maybe small diesel sports cars. Ideally, I guess I would've preferred a 5w40 rated fully synthetic HDEO like Mobil1 TD or Delvac1 or even Delo 5w40 which chevron Pakistan doesn't sell. Have a look at this post I made at BITOG:
https://www.bobistheoilguy.com/forums/ubbthreads.php/topics/4751720#Post4751720
And if you go through other threads too you'd see almost everyone recommends an HDEO over an expensive SN/CF.