If S&W doesn't want to offer 30 round mags for the M&P, then that's their business. Mec-Gar could do it, of course. I wouldn't want a Pro-Mag. Had enough of them 20 years ago to know they're trash.
All I know is I haven't owned a Glock in 20 years. When I got the Ruger PCC-9, I immediately switched out the magwell for the Glock one. I also bought 3 33 round Glock mags (just because). I also bought 3 10 round Glock 26 mags, and several Glock 17 mags. And buying all of those mags has made me consider having a pistol to pair it to, specifically a Glock 19 or 17. And if there were quality 30 round mags for the M&P, and a mag well insert for the PCC-9, the Glock wouldn't even be considered. Making 30 round mags doesn't require research or development. It's a steel tube and a longer spring. Just seems dumb to not invest the very small investment for an easy return.
But I guess I'm a "child" for liking the versatility. And I guess I don't see the difference in offering a 30 round mag option for a rifle, when they ship every M&P 15 with one. Seems to me 17 rounds in a .223 is "enough", as well. And do you think that just maybe Ruger included a Glock adapter because it would be a drawing point to those who not only owned Glocks (NOT like myself), but to also include those who wanted the option of 30 rounds in a home defense or sporting Carbine (like myself)?