It's not so much the ease of destroying the HQ, but the effect that is the problem. Why should one person yield the power to destroy the single most important target in the game? The damage threshold should be increased, so that it takes 3 loaded sets of Lancs (or equivalent) to get the job done.
But then, the main problem still persists - darbars. It seems like the only benefit to destroying an enemy HQ is that the opposing players either log off or have to spend their valuable game time resupplying and not in combat with other players (the whole purpose of the game).
The HQ getting destroyed should result in a significant benefit to the attacking force, but not one just to piss the other side off.
Suggestion for an alternative to the current loss of darbars:
HQ destruction results in all auto-acks (fields, towns, strats, CVs) disabled until HQ repaired.
This is my favorite option because it encourages the enemy to attack even more targets. Everything will be more vulnerable and this gives a big incentive to spread out, not just pile onto a single base. More fields under attack gives defenders a better chance at smaller fights, rather than fighting the steamroller.
Stagnating maps can be given a good nudge out the door but the small maps could get rolled too fast, requiring the implementation of the "24 hour minimum map rule". This means any map reset within 24 hours gets reset to the same map, so that everyone gets a chance to play on the maps they like.
The moral of the story: nobody would need to fly goons to resupply, just so they can find out where the action is. HQ going down wouldn't discourage interaction with the enemy, quite the opposite in fact.
