The point you seem to be missing that is that ignoring the fighter's mark assists the fighter in killing the monster. The fighter is sticky not because enemies can't shift+charge away but because doing so can quite expensive since it gets you beat down faster. It can be really dumb; sometimes giving the fighter double damage!
Sorry, but this is a widely believed fallacy.
What you say is true if one doesn't look at the encounter as a whole. But, it's false if one does.
One or two or even three extra attacks in an encounter, successful or not, by the Fighter doesn't mean that much in the large scheme of things if the enemies take down a Striker or especially a Leader in the process.
The main advantage that the PCs have over the NPCs is healing. If the NPCs take out the Leader, that advantage is wiped out.
A secondary large advantage that the PCs have over the NPCs is striking. Very few NPCs have really good striking capability like Striker PCs do. If the NPCs take out the Striker, that advantage is wiped out.
The role of the Defender is to minimize the number of times that enemies can attack the other party members. Doing a little extra damage in an encounter rarely accomplishes that goal. Forcing or encouraging the enemies to attack the Defender instead of someone else is a lot more effective in accomplishing that goal.
Moreover, from an RP in world perspective, most monsters will prefer to let the other monster shift away first. If both marked guys are waiting for the other guy to disengage first, then it's probably not going to happen right away. The first guy taking the ideal (assuming you think it's worth it to take the attack in the first place) group action suffers a not ideal personal consequence. This is by no means an insurmountable problem, but that doesn't mean it won't be a problem for many monsters.
You have this backwards as well. The monsters should not be aware of Combat Challenge until it actually occurs. By definition of what you wrote here, the monsters are aware of Combat Challenge. They shouldn't be. They should only be aware that they are marked.
From a RPing perspective, shifting away from the heavily armored and marking PC to then go attack lighter armored PCs (and especially a PC that is healing) is very desirable. So, the first marked (or even non-marked) monster should want to shift away from a PC that did a mark.
He pays the price of Combat Challenge. That is the first time Combat Challenge is observed in the combat.
But, Combat Challenge is an interrupt. Just like the PCs know when a monster does an interrupt and can no longer do another one right away, so should the monsters know that the Fighter cannot do another interrupt immediately (in reality, the players know and the DM knows, but the net result is the same).
This frees up the board at that point for the rest of the monsters to shift away from the Fighter.
Granted, monsters might be more wary of being the first foe to shift away in later rounds once they have experienced Combat Challenge, but not until it has actually been seen. Course, a strong NPC Leader type should often order lesser NPCs to "stop messing with that one, attack the healer" or some such anyway (for those monsters that can communicate, monsters should be shouting out tactical stuff, just like PCs do).
Finally, the group can adjust its tactics and abilities to suit the situation. If people have a problem with rational monsters ignoring the mark and pounding more vulnerable characters, then change something. Increasing the fighter's offense at the expense of defense (ie, not using a shield) makes ignoring the mark more expensive since the fighter hits harder, and makes attacking the fighter a bit more attractive since his defense is lower. Similarly, taking more defense feats on the other characters means that dealing damage to them is somewhat harder, and also potentially somewhat less rewarding if they give up some offense. Continue adjusting until you reach a comfortable equilibrium...
Yup. No doubt.
Here we agree. The players should adjust tactics based on the tactics of their foes. Even in the middle of an encounter. The Fighter drops his Shield and starts using his Longsword two handed is a perfectly viable tactic mid-encounter.
But, the player will never do this if the DM does not play each monster to the level of its abilities and knowledge. The game becomes more repetitive and predictable if most of the monsters do not ignore the Defender(s) and target other PCs when appropriate and possible.
And all of this should be based on monster knowledge, not DM knowledge. The Warden with a low looking AC in Hide armor might be an early target for monsters until he starts marking them, etc. But most intelligent NPCs should try to get away from PCs that mark them by definition. A marking PC should not often be considered an easy target, rather he should be considered difficult.