Why bother gearing up in Heavensward when quests in the expansion is going to replace that gear too? So you don't go and get yourself new gear for your alt classes (gear that you should hopefully be able to turn in for GC seals when it does become obsolete, so you can get a small head start on THAT compared to people without the gear.) There's still other stuff to do. If none of it interests you, and you're only interested in end-game raiding.. then you're playing the wrong game. You should be playing Wrath of the Lich King. :\ Ulduar was the best raid I've ever played in any game. Nothing else anywhere has come close to it, but I can't keep holding my breath waiting for something to beat or tie Ulduar, because nothing else can really have that same 'magic' without it being called a ripoff of Ulduar, or a cheap clone of it. It's almost impossible to better THAT experience, so I just went and explored other games after my interest in that finally died down.
Wanting content to last, and hating that things go obsolete in 3 months... that logic is completely against itself. You can't have things last and then get new shit faster than 3 months. Content takes time to create. It takes infinitely less time to do it and make it 'obsolete' than it does to create it in the first place.
Alexander will be the HW end-game raid. Normal and hard modes with it. (Basically think normal coil, and savage coil...) but as for it lasting more than 3 months? Very unlikely given how quick players always chew through things. On the other hand though, you can't have 500 hard raids or else people just wind up overwhelmed by all of them. Or they wouldn't be balanced. Or that not enough players would get to see all of them. Or that all the gear coming from them is basically identical to one another stat-wise, so why do the harder things when the faster ones get them the same gear. Or any number of a thousand reasons..
Yes, there probably does need to be more for the 'hardcore raider' crowd. But that type of player (and not the thousands of people who call themselves 'hardcore' and live vicariously through the real raiders, or the people who beat the hard stuff 2 months later and think they're hot shit) is in such an extreme minority that there will never be anything that can be done to really please them.