One reason Narita works so well (and similarly, AMS) is that the connection for passengers not staying in the hub is fairly painless. For NRT, you do a simple security checkpoint, not unlike what TSA PreCheck gives you where no liquids are removed and you can leave your shoes on. It makes NRT a fantastic connecting hub and allows (allowed?) NW and now DL to funnel many people from multiple points in the U.S. onward to multiple points in Asia very efficiently. I've been known to be off the inbound flight and onboard the connecting flight within 20-30 minutes -- granted that was due to a delayed inbound flight, but it highlights how efficient that hub can be. I wouldn't even expect that level of efficiency from a Delta hub here in the U.S. Narita just works so well when it funnels ~25 flights per day or so in and out.

For AMS, unless you're connecting to the EU, you have no customs/border patrol at all, and no security until (possibly) at your connecting gate. Even if connecting to an EU flight, the process is pretty painless.

Now, PVG is another story, since there is no easy way to do an airside connection without actually entering China. They're just not set up for that (yet?). There are transfer desks, but they're rarely staffed, and when they are, in my experience, you're directed to simply "enter China", anyway, through the normal customs and border patrol process and then proceed back to ticketing or immigration as if you had already been in China and were departing.

That process would need to get fixed and made similarly more efficient. If DL wants to abandon NRT for PVG, connecting its passengers there instead of continuing the point-to-point trend they've started from DTW and now SEA, they have to make it as efficient as NRT or AMS is, and that won't be easy, or quick.

For starters, Delta, China Eastern, and China Southern all need to be co-located in the same terminal.