It was removed as part of the "casters should stand and cast" bs that they have put into the current meta, while seemingly forgetting to prune all the gazilion gap closing tools from melee.
The dk is actually very mobile now with wraith walk being pretty good, the warlock is now the least mobile class, with their only speed boost being a talent... in the same row as one of the best defensive for lock...
I'm not sure where Alter Time was really "iconic" to arcane or really any mage school.
But I agree. Losing temporal shield hurt, losing alter time hurts more. I liked when they modified it in WoD to clarify it's intent to be used defensively. And as for blazing speed, my only answer to that is. I'm a mage. I'd much rather blink away and get back to shooting faster than trying to walk-er run away and risk tripping over my really fancy cloth robes that do absolutely nothing to protect me from bodily harm. Of course the fact that melee will just gap close constantly with more tools than we have openers is ... well... bleh. But they can never really balance it for PvP unless they make all spells instant cast to match melee abilities just with longer CD's, or make all melee abilities have a cast time so they can be interrupted by movement and the like.
The thing I haven't seen mentioned is if we are getting Momentum Glyph baked in or if it too is gone entirely. Took a while to convince me to give it a try, but once I started using it I don't ever want to go back.
It wouldn't be so bad if melee specs actually lost their eight gap closers per spec so when you blinked they had to run over to you instead of instantly being on top of you again.
I'll really miss Alter time, I've come to rely on it so much both for movement and as a defensive. I wish they would keep it around in some form for the hp. Revert your hp pool back 6 sec or something
Why does something have to replace something?