The main reason, unless I'm mistaken, that EK are better of attacking instead of casting spells (aside from spell slots) is that you only get Extra Attacks if you take the Attack action.
As of yet, we don't know how extra attacks will be implemented. I personally, based on current implementation of Haste Attack and Bonus Action Attacks, think that Extra Attacks will be implemented as extra "action points" which the Fighter will be able to spend on any Actions of their choosing. If my hunch turns out to be correct, this means that the EK's "opportunity loss" from casting an Action-costing spell will be much lower, as they will be able to cast a spell and then use their remaining "action points" to attack, or vice versa.
It probably still won't be more useful than just all out attacking (especially because of spell slots), but it will be more powerful than book EK.
That's a scary thought.. I honestly hadn't realized the potion of speed was so broken.. I have rarely ever used it because I never needed to and didn't want to waste it.
On the other hand.. that's potentially a nerf to extra attack for fighters since action surge would then only give one extra attack not two. Hmm.