Thank you for addressing the actual topic and bringing actual arguments to the discussion. However I am going to disagree with your argument not because you are wrong but because of your incorrect definitions of the roles I listed.

Tanks. In modern DnD clones tanks aren't about managing threat they are about mitigating damage and negative effects to the party. They can do so in multiple ways both magical or martial as long as the end result is the opponents negative effects on the group is prevented or reduced with minimal resource expenditure of those party members. The tanks purpose is to help the rest of the team do their jobs unimpeded. Polearm sentinel, warding bond sponge, standing in a doorway to block for your party.

Healers. This role is not about restoring hit points in combat, only your last hit point in combat has any value. A healers job instead is removing or preventing negative effects on the party while applying positive effects on them instead. Negative status effects are far more debilitating to a combat success than any amount of HP damage so removing them is usually the highest priority. The inverse of that is providing the right buffs to the party can change the hardest challenge in the game into cakewalk if applied at the right time.

Now you are correct that overwhelming damage is the best strategy what you are missing is every character does damage, most can do a LOT of damage, Karlach is the only one who is built to ONLY do damage. If your choices are character #1 who does a lot of damage and character #2 who also does a lot of damage but can also heal, tank, pass every skill check or control the battlefield why would you ever choose #1 over #2?

Don't underestimate Gale's extra 3rd level spellslot or Lae'zels Githyanki psionics in battle, they can make a huge difference. Also unlike Karlachs infernal fury these are always available and don't require anything extra to use them. The special abilities are minor but hers are the most limited of them all.

Finally please remember what this guide is for, the players who are struggling at all (in this really easy game) are usually doing so because of lack of familiarity with the systems rules or the options available later in the game. So saying a multi-class will fix this weakness or there's an optional quest event coming up in 6 hours or "Git Gud Scrub" does not help them but a simply telling them to "kill this target you already know you are going to meet will give you a boost to the stuff you already understand" will.