Several players have carried out an action similar to the one mentioned, "raise all the defense to the maximum and sell the buildings to improve others".
As has been repeatedly warned in this forum, in the announcement of the version that introduced that change and in several YouTube videos, for a long time (years?) you need to have defensive buildings to have defense. If you create the defense and do not have the defensive buildings, the defense units will flee as soon as the base is attacked, leaving them completely defenseless.
1) We must not assure that it is a failure of the game if we do not know the rules of the game and, therefore, it is not a failure
2) No, I am not happy when a player loses the effectiveness of his defense
3) Yes, to use the defense again you have to have the buildings at maximum capacity again (and that is your responsibility)
* Another typical mistake is to create level 65 attack units and then sell the command center, so you can never attack even if you have your attack units at the highest level.
Note: Perhaps it would be interesting if a poster appeared when the player asked for the sale of an important building indicating the consequences (the truth is that I don't know if they already appear because I don't sell those important buildings).
· If you sell a defensive structure: "Your defense will disappear and will not be effective until you have the buildings at the appropriate level"
· If you sell the airfield: "Your air attack units will not participate in the attack and will not be effective until you have the buildings at the appropriate level"
· If you sell the factory: "Your ground attack vehicles will not participate in the attack and will not be effective until you have the buildings at the appropriate level"
... and so on
