I can use frost spells, frost nova, frozen ring and ice block, so it's non-issue (if you want to continue this cherry picking you should go with "arcane can't use fire spells!" and i already tired of this argument, i've been talking about it for years - it just doesn't make any sense for someone heavily specialized into fire magic to use frost magic, if they can do same shit with their favorable element).
It's a game, and for gameplay purposes we have an ability to respec (if you wouldn't be able to respec - a call for making specs - classes, actually would have at least some sense in it, but still it wouldn't matter), specializations went from "small tweaks to how your abilities work" to "you get a whole bunch of new tools that make your old tools obsolete or OP as fuck in combination with everything else, so we take it away and/or alter it to fit the theme more". Which is fine, and doesn't make specializations "new classes" or whatever
Back in vanilla arcane wasn't a "spec" if you want to call it, it was an augmentation tree, you dump a lot of points into it and get better in both fire and frost.
It couldn't be another class, because it's a spec of a class. If you imagine classes in games to be as bland and boring as singular specs - well, i have nothing to say, classes should be different both gameplay and fantasy-wise, having, basically, 3 similar classes is just stupid.