In RPG-skirmish style combat, where effectiveness is simplified to "fully-functional until dead", a fundamentally powerful strategy* is removing participants one-at-a-time from the combat. Focus-fire enables this. OAs do not.
I feel like this is being overlooked by your thesis. The putative Catch-22 is countered by a more powerful Catch-23.
*...and a good argument for critical hit systems that incrementally cripple participants.
So here is the elaboration I promised. You have already agreed that looking at a fully ranged party is pointless. So for right now we are looking at a party with at least 2 melee PC's and at least 2 ranged PC's.
The tactic I'm proposing is that the ranged PC's still focus fire on the target of one of the melee PC's. Assuming team PC spreads their melee characters out as previously described then at least one enemy will take at least 1 OA if they attempt the focus fire strategy.
If the enemies do take the OA to focus fire on one of the melee PC's then there is now nothing preventing the PC's from focus firing back as all the enemies are clumped around 1 melee PC and OA's no longer deter the targeting of that PC. That's where actions like the dodge action start looking very impressive. When all the enemies are attacking a single PC then having that PC dodge (or some defensive spell cast on him start looking really impressive).
So looking at the 2 melee 2 ranged PC's after round 1, the focus fired enemy will have taken 3 attacks (2 ranged 1 melee) and another enemy will have taken 2 attacks (1 melee and 1 OA). 1 PC takes 4 attacks.
After turn 1 and everything has converged and started to focus on the 1 PC then the rest of the fight will be actions to keep that PC alive and actions to focus fire down the rest of the enemies. If enemies only need 4 attacks to kill then turn 2 I can kill 2 enemies with 3 attacks which gives the focused PC the option of taking the dodge action, or of some other party member of defensively supporting him.
At the end of 2 rounds 2 enemies have died and the party has taken 8 attacks. 4 presumably without any additional defenses and 4 presumably with dodge up (or the other melee PC possibly was targeted instead).
Your focus fire only strategy would potentially have 7 attacks targeting 1 PC after 1 round and none of those attacks would be facing the dodge action.
Even in possibly the best case scenario for "PC focus fire of enemies (when 1 round worth of attacks is guaranteed to kill an enemy)" my basic tactics for spreading out or mitigating damage are looking superior to the simple always focus fire mantra.
Consider if in the above scenario it only took 3 attacks to down an enemy. Also consider if it took 5 attack to down an enemy. Both of those cases highly favor my focus fire and attempt to spread damage tactic much more than the always focus fire tactic.
My tactic will cause the team to take more attacks but the individuals on the team are better able to survive as the damage either gets mitigated easier or spread around easier.
That's a simple 2 ranged vs 2 melee group comparison against 4 enemy combatants.