I'm not so sure. Simply taking an attack of opportunity and walking away will serve the same purpose, and fighters are good at doing that.
That's somewhat true, but especially at higher levels that becomes less of an option. Consider a CR 9 Fire Giant, for example. At +11 to hit, and dealing 6d6+7 damage, that's not necessarily a great trade-off for an extra 4d8 damage. Granted, the Fire Giant won't always hit, but even 50/50 odds (roughly speaking) aren't that great in my book (especially once you factor in the risk of the odd crit). Plus, if you leave him unattended, that Fire Giant can just toss a hard hitting rock at your casters, which could disrupt their concentration.
Compare that with the Battlemaster's Goading Attack, which can be used with the same tactics or even at range. While it doesn't add as much damage on its own as BB, it also doesn't restrict the number of attacks a fighter can make. It significantly decreases the giant's chance to hit, and cuts the risk of a crit twenty-fold. Assuming proper terrain, a goading archer can easily make it so that the giant cannot attack anyone without disadvantage.
Given that the ideal outcome of an encounter is to overcome it with the least resource expenditure possible, Goading Attack is likely to be a better option than Booming Blade in many circumstances, at least in my opinion.
Frankly, walking away simply isn't a great choice for a tank is many situations. Between your opportunity attack and Booming Blade, you can likely guarantee that your target will remain focused on you (assuming no one else is in melee with it). If you walk away, it can just go after a softer, more opportune target. Admittedly, not every Booming Blade user will be a tank. However, if someone were to skirmish against one of my monsters using BB, I'd take the damage once to have the monster move to a less mobile target and that would be the end of it. An extra 4d8 damage once per encounter is nice, but not all that amazing at 17th level.
My point is how would a cantrip that immobilized the target be priced? Compare "ray of frost" to "firebolt". But, with me looking at the EK, I am comparing a cantrip plus a class ability.
Admittedly, I don't think that a cantrip should ever immobilize a target outright. IIRC, an early playtest version of Ray of Frost did that and it was highly exploitable. That said, offering the choice between immobilization and damage shouldn't be priced nearly as highly, in my opinion, because the DM can choose the better option based on circumstance. In the case of the Fire Giant, the DM chooses to throw a rock at the wizard rather than take the damage. In the case of a wyvern, he takes the damage and gets to make all his attacks at the target of his choosing.
Frankly, I wish that more spells offered this type of choice. I think it makes things more interesting and dynamic.
