A few thoughts:

1) I'm okay with the proposition that magic sometimes puts a mage outside of a Rager's grasp. Of all the ways players have found to taunt or "grief" each other, this doesn't even make the first page in my opinion. Nobody can force you to use a portal, let alone multiple times, and if a mage uses one escape then good on them, and if they use it to sit and hide then they're most likely making a trade-off to do so.

2) Most portals, like the one you used as an example, result in zero lag. This has a couple of implications that make the scenario you described a lot less problematic.

3) Trolling Ragers and/or being effectively safe from them has always been possible for a clever mage 99% of the time via pass door, fly, or other less widely known tricks. This has never been a real problem. Yes, there are things a Rager could do some of the time or against certain opponents, but that remains true in the case of portal sickness as well.

4) IF (if!) someone actually employed a trigger to abuse this mechanic, and there is only a small subset of places it would be advantageous to do so, I will happy dispense with them.