Is it okay for a DM to repeatedly attack a PC with an effect that removes him from the combat? Or should he spread that attack around? Obviously I believe the latter. ;-)
You seem to want everyone to agree with you. Personally, I do not. I think the DM should run the monsters as creatures that want to survive, not stupid creatures that want to die.
Your answer is: The players are not having fun if they are taken out of the encounter, hence, I should have the monsters help out the players in this area.
When you do so, you are encouraging your players to be lazy and haphazard in their decision making. They will not bother to try to come up with interesting and intelligent solutions to tactical problems because big brother DM is there to catch them in his safety net, all in the name of fun. All praise be its name. Amen.
I have played with many hundreds of players over the decades. The one thing that many of them want to do to have fun in combat is to shine. They want to roll a critical and do big damage. They want to save the party. Some of them are just happy when they hit.
But, a lot of them don't want the DM to hold their hand and protect them from themselves. Give them a clue or help them out a bit out of combat when they are stuck to move the game along? Sure.
Hold their hand and make sure they never lose an encounter or make sure that they are never incapacitated for more than a round or fudge dice or monster decision making in their favor? Not many. Some players might want that, but most want to solve their own problems in combat.
However, it amazes me that other DMs would hold the enjoyment of the experience that their players have in such low regard.
No, we are not doing that. We are advising you to help your players be better tactical players. A discussion after the game where the subject of delaying comes up will spark ideas within the minds of your players.
A discussion after the game where the subject of how much being unconscious sucks is counter-productive.
As DM, your job is to help facilitate fun. But fun comes in many flavors.
As a player, I have a lot of fun just hanging out and doing my job. But every once in a while, the chips are down and my allies are in trouble. That's when I have the most fun. I pull out some tactical combo and re-right the ship. This to me is fun. In your game, if I ended up being the dog food in front of the monster, I too would be frustrated. Not at the DM for playing the monster smart, but for playing a sorcerer too close to the monster.
Players learn from both good and bad experiences. Back in the 2E days, I was playing a Ranger. My Ranger and an ally were holding a 10 foot wide corridor against medium sized troll-like creatures. We had knockback house rules back in those days (similar to push) and my ally got knocked back 5 feet. I could have shifted back to reform the line, but didn't think to do so. So, the trolls shifted forward and instead of it being 2 on 2, it was suddenly 3 trolls against just my PC. He fell quickly with 3 creatures doing claw, claw, bite on him.
If the DM would have not done the smart move for the trolls, I would not have learned a valuable front line lesson that day. I appreciate the fact that the DM played the monsters intelligently because I learned something that has helped me in encounters to this day.
If he would have said "Well, this will probably kill the Ranger and then the player will not be having fun because he cannot play at all until he gets a new PC, I shouldn't do that" then I would not have learned that particular lesson as well.
That one lesson has helped in many different ways over the years. It doesn't just apply to front line fights. It's a lesson in controlling the squares around the PCs and the monsters, and applies to many aspects of an encounter.
You should have discussions about this one frustrating encounter, but the lessons and conclusions that you take away from it have should have nothing to do with the topic of how often the DM stun-locks or unconsciousness-locks a single PC. It should do with how the other players should act and react when the DM does it the very first round. That's the important lesson. It's one for the players, not one for the DM.
Your conclusion is that the DM should adjust his behavior. No, he did the right thing. The conclusion should be that in order for everyone to have fun and the party to function better as a whole, the PLAYERS should adjust their tactics when that happens.
The number one lesson here is: It is the job of the players to help out other PCs. It is the job of the DM to harm the other PCs. Shift responsibility here to the people at the table whose job it is to minimize other PCs from being taken out of the encounter. That's NOT the job of the DM. His job is not to help the PCs out in a fight. That's the job of the players.