OLF, a sniper with a bow can kill you from over there, in the game as is, so could a caster with a single spell.
That's again, totally not the point.
The sniper is in the tower 10 blocks away, he fires once. . .he's still in the tower. . .you can go to the tower, you can shoot back at the tower, you can hide, combat resolves.
The mage is right on top of you with minimal or no range penalties to make his three attacks, then is gone. . he could teleport to another continent on the way out. . .what are you going to do about it?
Forcing the mage to one spell a round means he teleports in, you might not notice, he fireballs you, you notice and react, he can fireball you another 2 times if he wants to give you 3 rounds to find and counterattack him, then teleport out on the 5th round.
Which is not the same thing as the mage teleporting in fireballing you thrice and teleporting out inside one round. . even if you notice the mage, you have almost no time to react before he's gone and completely outside your ability to counterattack.
The plane attacking from over the horizon is firing, waiting a bunch of rounds for the missile to reach you and go off, and again, is still roughly in the area, not instantly back in it's hanger on another continent so the pilot can sip a beer with impunity.
The issue is not about unstoppable levels of force, hell, a dragon vs 1st level PCs is unstoppable force. . .it's about allowing someone to string an action combo inside a round that always starts with a nearly unstoppable ambush and ends with them escaping to nearly complete safety. . .within the round in which the target is being surprise attacked and likely gets no effective counter off.
You need to work hard to get yourself into a situation where you can ambush and then escape, it shouldn't be something a caster can do casually at will as long as they don't roll an UM 01-02.
That's the core issue. . it's not what combos make possible, it's what they make CASUALLY possible with little effort beyond "ABBBA combo seems like it will work.". . .I think it's game braking to make something that nasty casual. . .
Getting into the right tower, at the right time, with a rifle, without getting caught, hitting your target then getting away is not casual. Getting a plane with stand off attack capability to within attack range, with accurate spotting of a non line of sight target, without being detected or tipping of the target, hitting the target and then getting away also isn't casual.