Wasn't it strange, that you had the possibility to kill a mob, that dropped a cloak way better than any other currently in Frostmourne?
I have tailoring and I can tell you something about the two epic cloak recipes. Those require a huge time investment and a lot of resources - one of them wants the tailor to do the LOREMASTER achievement for Northrend, while the other requires Northrend dungeoneer. Could you imagine how much effort it would take to obtain them? And their ilvl is only 200.
Now let's talk Naxxramas. Think about the boss, that drops your BiS cloak. Think about the bosses you have to kill prior to him. Think about the BUFFED mechanics you have to avoid. Think about how hard it is to assemble a group of 25 people and to distribute the loot among the players. Wasn't it strange, that a cloak from a 5-man dungeon event was easier to obtain and it offered an upgrade over a raid cloak? Who could have participated in that event? Everyone that were lvl 80? How many 80s have the possibility to enter Naxx raids? Exactly. Now, knowing this, it should have been obvious, that the upgraded gear wasn't intended and the devs would have done sth about it sooner or later. Yes, they made a design mistake by implementing a loot table, which wasn't suited for the current patch. But such accidents are to be expected in a seasonal server with 3.3.5 talent trees and 3.3.5 locked instances. Also, bugs occur.
If I were you, I wouldn't have sold/disenchanted my old cloak knowing, that something was iffy. I admit I would have also used the new cloak, but I would have expected it to last until the end of the event, not permanently. There was no real reason to get rid of your past cloaks. The logic you try to defend is only applicable, when the said item is planned to stay in the game. You got screwed, because YOU ASSUMED it would stay. No one forced you to sell/disenchant your old cloaks, you did it on your own.