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 (Gen1), 19 (Gen2), 21SF (RTF2), 26, 39
Glock Armorer (AC), Glock Instructor (IW)
Glock Advanced Armorer (AA)
Remington 870 Police Magnum, Benelli M1 S90
Smith & Wesson 13-2, 16-3, 36, 66-3, 681-3, 3000