I mostly agree with what you're saying here, but your 3rd point is a tad disingenuous.
- Naxx was more or less recycled, but to me this is okay because very little of their population saw it when it was relevant.
- Onyxia wasn't really "content" for the last expansion that was supposed to mean anything. It was literally a bonus raid added in celebration of their 5th anniversary.
- The two (yes, only two) 5 mans that got "recycled" are only the same instance in name and layout only. The content inside the dungeon (you know, the part that counts) is completely different. The bosses are completely different, the drops are completely different, the trash is completely different. They are effectively brand new instances.
- Likewise, the 5 man Zul'Gurub is literally nothing like the Vanilla Zul'Gurub raid. Zul'aman is pretty much the same though. However, these two instances were patched into the game. The 4 instances added in WoLK after the expansion's release weren't exactly shining examples of creativity, either. Before that? In TBC and Vanilla you simply did not get new 5 man instances patched into the game.