Kahr has had single-stack polymer 9mm guns available for years. Good guns. The Shield doesn't break any new ground. Glock knows it's market - service-caliber weapons that are mag-compatible. The G26 is a better gun than the Shield. Care to do a durability test? Glock has nothing to prove, and the G26 is a bullet hose.
S&W did a polymer-frame single stack 9mm about 10 years back (SW9M, anyone? More like "S&M"!). It went - exactly nowhere.
J-frame and 158gr Winchester or Federal LSWCHP+P. If not that, carry something worthy of a belt holster - like a G26 or G27.
Remember, folks, S&W's selling point is that the M&P is now the "anti-Glock". Count on S&W to offer what Glock doesn't. Know what? I hope S&W succeeds. Why? Because LE needs guns with S&W features (manual safety levers, mag disconnects, and even single-stack service caliber guns). The departure of S&W's metal-framed 3rd Gen/TSW guns left a vacuum. It's filled now.
When S&W (or anyone else) builds something better than a G17, G19, or G26, they will earn my undivided attention. Getting closer, but no cigar, Springfield. I still may own a M&P 45 someday, though. Glock does nothing in "ACP" that makes me happy.
Glock 17, 19, 21SF, 26, 37, 39
Glock Advanced Armorer (AA), Glock Instructor Workshop (IW)
Remington 870, Benelli M1, Smith & Wesson 3000, Mossberg 500