Counterspelled Booming Blade

[diplomancer]

Incorrect. Making a melee weapon attack takes an action: the Attack Action.

I fail to understand you. Making the melee attack is not the Action, the Attack Action is the Action. And when you take that Action, you can make one weapon attack (or more if another feature allows it).

That is the general rule. It is only by exception (two weapon fighting rule for example, Specific over general) that making a melee weapon attack is handled by another rule.

Precisely. And Booming Blade is one of those rules that create an exception to the general rule "if you want to make a weapon attack, you should take the Attack Action", but when you do decide to use this cantrip, you're not taking the Attack Action, but the Cast a Spell action. If you allow the melee attack to take place even if the spell is counterspelled, you are effectively allowing the caster to switch from the Cast a Spell Action to the Attack Action (but one attack only always, kinda like the hasted Attack action), AFTER he had selected and used the Cast a Spell Action.

As to: Steel Wind Strike. That is a 5th Level spell, and not appropriate for a discussion about a cantrip.

Why exactly? In both cases, you're taking the Cast a Spell Action, and taking this action allows you to make one weapon attack (in the case of Booming Blade) or five spell attacks (in the case of Steel Wind Strike), and Counterspell stops both spells equally. Cantrips are spells.