I usually gravitate away from single stack/ultra slim 9mms. I'm not big on iwb carry, so the only single stacks I keep in any kind of rotation are the Beretta Pico in my pocket or the 1911 owb on my hip or shoulder holstered.
I used to have a 26, but I found that since I owb carry 99% of the time, the 19 is a better fit for my needs.
I see the Shield as a fine addition for those looking for a slim 9mm where iwb concealment is of high priority. I could certainly see the Shield pulling duty when discretion is key and a 26 bringing up the rear when it can be concealed.
For me, a small .380 works for an "invisible gun". I can dress around a 19 most of the time. Thus, I don't have much demand for either the 26 or the Shield.