ORIGINAL: Harrybanana
All I can say is that this is what happened to me in my game with Yeremenko (which I have now resigned). In fact, as I said it happened twice, first to a full strength Army in Portsmouth (which I think was only entrenched to level 1 or 2) and then to a full strength Corps in Bristol with maximum entrenchment. Admittedly I don't think either unit was attached to an HQ. But the unit in Portsmouth was defended by an AA in an adjacent hex and both Cities had their AA upgraded to level 1.
You say you attacked with 7 TAC, but in my game my opponent first attacked with 2 (or maybe even 3) Med Bombers to reduce the entrenchment and efficiency of the defending unit. Did you try that? The usual MO is to attack the enemy unit first with corps or Med Bombers to reduce the entrenchment, then finish it off with TAC.
Thanks and I just re-ran the test starting with 2 Medium Bombers, also with 1 experience point and Level-1 Ground Attack applied and also attached to the same HQ that has 2 experience points and then followed up with 5 Tactical Bombers instead of the 7, and on a first try was able to bring the Corps down to 3. Granted it could have been possibly knocked out with more favourable +/- modifiers per attack, but the 3 seemed an average result as it was +1 on a few attacks and -1 one on the others.
The defending Corps is attached to an HQ, and for the test I ran the Corps of course starts at 90% readiness and 100% morale. On the subsequent turn, after having its morale completely reduced from the previous rounds attack, and even though it started back at strength 10 after having been reinforced, it was much more quickly reduced and only needed half the attacks.
So on the surface it feels reasonable enough in the sense that an attached HQ for the defender, and morale and readiness are playing a big part in the combat results, and that after a few rounds of concentrated attack the unit would eventually be no longer able to withstand the withering assault. I just say this as it took arguably considerable effort to reduce the unit to 0 after two turns, but of course it started out optimally as well.
Granted, in game terms, not every defending unit will start out as optimally, but if it does, it will take a very thorough and persistent effort to knock it out which again tells me we are ideally not that far off from reality (with how the current game mechanics are working as it is taking into account all the other factors as needed) even if defending land units are being destroyed by air units in the end.
So for me at least it comes back to whether such a concentration of air power should be further curbed, and remember some may like the ability to be able to do this as there are still pros and cons to this choice for the Axis after the USSR gets involved, and/or if defenders should be destroyable by air power alone.