I disagree with you big time.
If your base is maxed and you have pretty good walls, your defence IS INDEEED OP.
I mean, yes it can be destroyed, but it's by top players with top armies.
You gotta realize that 99% of player base can't wipe a maxed th10 base !
And it's just logical that with the good army comp (the most expensive ones with maxed heroes and stuff), your defense is likely to be crushed.
It's a normal thing (neither good nor bad) that as you go up in leaderboard, attack is what matters most because it's what makes the difference between players ("skill", army price and uptime).
But up to 2600, maybe even 3200 trophies, defence DOES MATTER A LOT. Try to get to champ league with crappy def of no defence : you'll have to compensate by attacking SOOOO much, using soooo much gems !
Defence / attack is pretty well balanced imo.
Max defence seems absolutly unbeatable for 99% of army comp / player base.
And yet, of course, when you use the most expensive units in the game (GoWiWi), defence get "crushed", even though it still has a word to say in terms of being two or three starred.
i think you lost sight with how "optimized" attackers are past 3200 trophies. Thus, it's perfectly ok for defence to become less and less meaningful the higher you go in trophies.
On the other hand, long hours of farm changed the way you can progress up to
2600-3000 trophies, making your base so hard to go against.