I'll bite here...
I have to agree on some of your points, and disagree with others. Unless you can not have a mag. that can have more than 10 rounds, I can not see why having a gun with a higher capacity, the ability to change grips, an ambidextrous mag release, and a rail which gives more options to mount a flashlight, lasers, etc, would be a reason way people would choose the Shield over the M&P9. I maybe wrong, but I don't hear to many people complaining about having those options and features.
All of the benefits of the 9C come with attending drawbacks, and as one person said earlier, those attending drawbacks are often the very reasons to choose a Shield.
Sure, more rounds would be great, if they were "free". But they're not free...you pay for them in the width of the gun's grip. Because the grip is wider to accommodate the double-stack magazine, the slide is also wider to match. It's not wider by a lot on paper, but it's a significant difference when it's in your pocket or in your waist.
The ability to change grips and ambidextrous magazine release, you would think, could be added to the Shield, sure. It likely wouldn't "cost" anything other than cost, as in money. To me, the ability to change grips is nullified if the gun fits your hand. It's a great marketing point to say that your firearm fits may different hands, but what matters to any given buyer is if it fits HIS hand, or HER hand. If it doesn't, they're likely not a Shield buyer. If it does, great...no need for different grip panels.
On the topic of a rail...many probably see it as unnecessary on a gun that is purpose-built for concealed carry. Unlike the 9C, which is a shorter and stockier version of the full-size 9, the Shield was designed from scratch for concealed carry use. Smith also partnered with Crimson Trace to launch a laser product with the gun's debut, so those who like lasers are covered here. Additionally, a rail adds girth to the lower part of the frame. Where the Shield has a nice smooth tapered contour, a rail "squares the corner" off and adds material that will likely never get used on a carry gun.
(I'm actually considering adding a Kel-Tec PF-9 to my carry arsenal, and it has a rail. I'd file it right off. I don't need it, and wouldn't want it.)
If all of the attributes of the 9C that are "more than" the Shield came with no penalties, then sure, nobody'd say no to that. But they do come with penalties. It's up to each buyer to weigh those out with what they're looking for in a gun.
I do agree that, in most cases, having the features and options that the shield lacks may not have been as important as to Shield buyers, but other than the size difference, I still believe the the M&P9c has more to offer.
Yes, the 9C has more to offer, in the sense that a Ford F-350 has more to offer than an F-150. More payload, more towing, heavier suspension, heavier frame, etc. There are a lot of things about an F-350 that are "more than" the F-150 in terms of quantitative measurements, but not necessarily "greater than" in terms of qualitative measurements. The penalties for the increased capability include things like a stiffer ride, poorer handling, higher price, greater fuel cost, etc.
I agree with you that the 9C could be said as having "more to offer", but it's really an academic discussion because that only applies to someone who values those particular qualities (round count, rail, etc). For someone who values compactness and ease-of-carry, the 9C has "less to offer", because to someone looking for a smaller carry piece, less really is more.
You could extend this discussion to comparing the 9C to the full size 9. "Why would anyone buy the 9C, right? I mean, yeah it's a little shorter, but it's just as wide and it holds a lot more rounds, right? Doesn't the 9 have more to offer than the 9C?" To some, it does. And to others, it does not.