...catching up on neglected maintenance and using tilting trains would also allow for substantial speed gains so that the net benefit of the new line will be 10-12 minutes.

Even worse: ICE trains in the 1990's used to be able to make the trip to Munich in 2:01. Today they need at least 2:20. S21 + Wendlingen-Ulm are supposed to bring the trip time back down to 1:58.

A study commissioned by the chamber of commerce of Stuttgart, a strong proponent of S21, shows the expected load of the existing lines by 2025 if everything is left as it is. Rail node Stuttgart (blue arrow) doesn't look very overtaxed.

I will disagree on this point, albeit no support for the Stuttgart 21 project as currently envisioned will follow from that.

If you look at that capacity map, you'll notice that while the Stuttgart node has no capacity problems (thanks to the extra S-Bahn tracks), the line to the East does. This line, a climb up the Geislinger Steige, is already heavily used by freight, local and express traffic alike, and at a slow speed.

An attempt to speed up long-distance service by using tilting trains (which are fraught by lots of problems in Germany anyways -- a good summary appeared recently in the June issue of Eisenbahn Kurier) would only worsen the capacity problem, because one fast train would increase the headways between freight/passenger trains before and after. And that's when trains run on time: when not, delays will cascade, freight trains will be forced to make lots of extra stops (especially not a god thing on grades), the faster trains will still get stuck behind late slow trains until the next station. In fact, from what I know, it's just this kind of delays that were the reason to lengthen Stuttgart-Munich ICE schedules.

With the additional consideration that a new line would also add capacity to enable drawing more passengers from other modes of transport, also on longer relations (Paris-Munich), the time and capacity factors lead me to follow that the Wendlingen-Ulm line would be a necessity. What doesn't follow is support for Stuttgart 21, or Wendlingen-Ulm according to the current specifications.

For much of the same reasons as you outline, I don't see any sense in forcing Stuttgart's rapid transit into the same tunnels and same station as the ICEs. As can be seen on your map, Stuttgart already has a tunnel for though S-Bahn trains. If this weren't a real estate project, half the access tunnels and half the underground platforms could be dropped, and the messy routing at the airport could go, too. Some further issues in a follow-up comment.

_{*Lunatic*, n.
One whose delusions are out of fashion.}