That depends completely on what your criteria are and what you’re training them to do when you say “use.”
SOME people can be trained to use DA/SA triggers effectively, but most folks have trouble with them and shoot better with consistent trigger pulls.
Three of the best trained federal law enforcement agencies (Federal Air Marshals, US Secret Service, and Diplomatic Security) would argue that point. Again, just because someone has trouble with it or lacks the experience to teach it doesn’t mean it’s bad.
It is different. As I said, Christmas geese notwithstanding, there are plenty of well respected instructors who simply don’t have the dedicated time with a TDA pistol to teach it well.
If shooting 3600 rounds in a few days, you probably can train someone to competence with the DA/SA action, however, I would bet that they would be more effective, faster with a single action or safe-action trigger pull.
None of the people in that class were starting from incompetence. 
So let’s parse your comment.
They would be more effective? Define that. If you’re suggesting that there would be a significant enough difference in skill level and performance that it would likely change the outcome of a fight, I challenge you to prove it. I’ve trained complete beginners and experienced shooters, and in ten years I’ve found without doubt that it is the shooter & instructor, not the gun which determines how much a student learns and how skilled he becomes.
They would be faster? How is a SFA or SA gun faster? Certainly the first shot can’t be faster, since they’ll both break at the same moment (as the arms reach full extension). Follow-up shot speed is going to be about visual speed and recoil management first, trigger reset second … none of those things have anything to do with a DA first shot.
A stock SIG, for example, with their “Short Reset Trigger” will have both a shorter reset and a lighter trigger pull for all shots but the first compared to a standard (“5.5#”) Glock. So how, exactly, will it be slower? How will it be less effective?
I’m not dumping on SFA or SA guns. I carry a Smith M&P, which is obviously a striker fired gun. But for someone to suggest that somehow a TDA pistol is a hindrance for a properly trained person is just silly.
Nonetheless, I do agree that TDA guns are on the way out. Plenty of people share your opinion and perception, and I wouldn’t be surprised if some flagship TDA agencies/units switch to SFA or DAO guns over the next year or two.
edited to add: I’ve never had goose for Christmas, and if they’re served full of fecal matter it might explain why some folks seem so cranky. Mom always made a tenderloin at our house. 