No duh the brand new thing has attracted a lot of attention. Wrath era will never have the population wrath prog had. Everyone asking for wrath era knows this and is completely okay with it.
Many people are simply quitting wrath now because there are currently no plans for wrath era. A different type of game you would have people trying to get as much play time in as possible before it goes away. For example, Heroes of Newerth (a moba similar to LoL) had a brief uptick in population when it’s impending closure was announced. WoW is different in that in addition to simply enjoying the active gameplay, a big draw is building up gear/achievments/HKs. The drop off in players of wrath right now is NOT a reflection of lack of interest in wrath era. If anything, it is the opposite because people who want wrath era as of now do not see any point in playing if their progress will be lost.
It’s a total bummer, the announcement itself is demoralizing to would-be wrath era players. If Blizz had stated they would support wrath era servers, I guarantee the current wrath population would be far higher.