Well, this thread got exciting when I went to sleep.
I'll try to reply individually, simply because one post that's replying to five people and five things at once can get confusing very quickly.
I would say that's tricky. If I take all my science and unlock defensive units, then shouldn't it hold out for much longer than MK I stuff? Wouldn't you expect MK I fleets to regularly fail completely, especially tackling higher MK worlds? Think of it this way, if you build a MK III world and send out MK I waves against a MK III AI world, what are your expected results? Should be something similar to what you do to AI waves at that level.
At this point I'd say AIP needs to be included in this kind of calculation, because early on , those defenses should hold just fine. I'd think they would hold well into mid-late game.
I'd say that's fair. What about during the HW assault? If your defenses are still impregnable at that point without any ship support, you've won. The only thing left to do is solve the HW puzzle (or zerg it, depending on how nasty it is). But there's no way to lose anymore. That's the condition we get in a lot of 4X games where the game is over but it takes 50 more turns for the game to realize that it's over.
To me, this comes down to a fundamental question of "what is the lose condition?" Is it strictly "you let AIP get too high?" If that's the case, impregnable defenses at the expected AIP is probably fine, since they should crumble once AIP gets high enough (although in Classic you can make them near impregnable, period).
Alternately, is "you threw away your entire fleet and the AI punished you for it" a lose condition, or at least a "you lost some planets and have to retake them" condition? I don't see how the AI is particularly threatening if it just sits there waiting for you even when it knows you have no means to attack it, instead of coming at you.