keelbreaker said:
I'm not surprised that you can. But it's not designed to be used that way and it's completely unreliable under controller range circumstances. When the adrenaline kicks in and actively shunts blood flow away from your extremities deadening dexterity. You don't want to be relying on that thing or planning to.
I've been using it that way in 30+ defensive firearms courses, with multiple full days of force on force, for hundreds of hours now of firearms instruction from instructors with a mix of military and law enforcement backgrounds. It can work that way and many of the instructors I have had do it that way themselves. If you don't want to use it that way, more power to you. I respect that by the sounds of things you're in law enforcement and no doubt have your own training background, but we have a lot of members on this board with quite a bit of experience as well.
The gross motor skills emphasis you mention from your instructors is one that has been popular in the past, but a number of instructors in recent years have relaxed on this. If fine motor skills are completely out of the window, then surely there's no reason to carry a spare magazine as I won't be able to press that magazine release (and as someone with stubby thumbs most of the pistols I own also require me to shift my grip to hit that magazine release, and not just Glocks). For that matter how are you going to be able to press that trigger smoothly enough to the rear in order to not throw the shot? There is a level of fine motor control that is needed in a defensive shooting. Now if the goal is to not rely on that when possible I get it, but my own experience and the experience of the instructors I've had is that slide stops or releases can be made to work, and honestly without any more difficulty than that magazine release or trigger.
As for why extended slide release are not the industry standard, my guess is that in part this is because they can cause more issues than they solve for some people. For a number of people with large hands/long fingers that shoot thumbs forward, the extended slide releases cause problems for them. This can manifest in those shooters inadvertently riding the slide stop, which prevents the slide from locking rearward after the last shot is fired, and the shooters then don't realize that the firearm needs to be reloaded. Other people can get inadvertent slide locks before the magazine is empty, where pressure from the thumb under the slide release locks the slide to the rear prematurely. I've seen both of these firsthand a number of times. I don't even have large hands and those issues are part of the reason why I stopped using an extended slide release as it would occasionally happen to me. Now an argument could be made here that practice and training can mitigate this, but so can practice and training mitigate being able to hit the standard slide release.
In my opinion, modifying the trigger to be deliberately lighter isn't the same as replacing one stock part with what can be another stock part, assuming you go with a Glock extended slide release. I generally agree with you that modifying a firearm is problematic. If your department won't let you modify the firearm (though I thought this was for your personal use) then it's a moot point anyway. But the argument that a person replacing one factory part with another factory part to aid in releasing the slide somehow paints that person as a "gun nut" eager to kill another human being is a stretch imo, even having read Massad Ayoob's arguments against modifying a firearm. Is it possible that argument could be used? Yes, but to that end so could replacing the stock sights be used against you, or just carrying the firearm in the first place.