The Desantis holster has no more chance of falling out of your pocket than any other manufacturer. I'd be more worried about the firearm falling out of the pocket.
I would worry much more about being able to get to the firearm period. If you had a 250# person on top of you, you may have some difficulty getting to your firearm. This is why situational awareness is so important. The last thing I worry about is the holster or firearm falling out of my pocket. If it is a concern, I wouldn't carry front pocket. Personally, I have deep pockets. I don't think my firearm would ever fall out of my pocket. I actually want as little retention as possible, for an easy draw. Kydex in my front pocket would be uncomfortable to say the least. Too stiff for my liking. I use kydex for OWB. Each to his own. I'll stick with my Desantis. I saw that a Remora was mentioned. A bit large for pocket carry, but the wife loves hers for her SP101. I only use my Desantis under certain conditions and dress. That's why it's so important to have many modes of carry.
My wife has an SP101 also. Used to be mine.
That getting to your gun with a big guy on top you would be a real challenge. I did enough ground fighting when I was young to know I don't want to be there. I totally agree about the awareness thing. I keep my cell phone in my pocket and my head up when moving about. The great thing about pocket carry is being able to get your firing grip without anyone knowing it. IMHO that is a priceless advantage. I don't pocket when driving because I have access issues. One other seldom mentioned advantage to pocket carry is when one is bush hogging with a tractor. Non-tractor people have no idea how much crap flies around when you bush hog. You get done an afternoon of that and you are about dirty as it is possible to be. I would prefer all of that stay off my pistol.