Yes, I agree, that's what the methods I outlined deal with. Foe is assumed to do the best thing for themselves. If that is attack the tank, then they are making less optimal attacks. If it is ignore the tank, then they are either or both of - giving the party an additional attack, and giving up an attack of their own in order to close.The enemies can do what is most beneficial for them unless you force them not to. If you have a pc built with really high defenses they can ignore him. If you have a ox with slightly above average defenses they can swarm him.
For sure you have to assume that intelligent foes, experienced in combat and whose lives depend on it, are doing the best thing. One has some control over what options are available. Remember that I am not arguing for slightly-better-than-average defenses. I am arguing for markedly-better-than-average defenses. Plate + defense at minimum. Or the tank is a battle master (menacing, trip, brace etc) and also carries a shield. Or an EK with shield and shield.
The question might be reframed as - given an average party of four, is it better to have zero tanks or some number of tanks? In my experience, parties with a small, non-zero number of tanks do extremely well. Parties with all archers can be very powerful, but can also find themselves suddenly dead.