This is clearly not the only incentive to do this raid since you can find healers and casters willing to do it.
People do not stop playing because they have nothing to do after buying their gear (you dont play content to get gear, you get gear to play content), they leave because its a private server and thats what people do on private servers, they just one day vanish into thin air. Ask any guild that has cleared SSC on the server, how many player from their first 25man are still raiding. Number is most likely not be higher than 10 (or even less).
At the moment its not a real issue because they are as many guild disbanding due to poor management (switch to 25man, having everyone progress on their attunements) than player leaving progressing guilds, so these guilds have raid ready players to chose from. But once you'll hit T6, this pool of player you can chose from will become smaller and smaller, because guilds that have been through T5 will likely stick around. So now you have players leaving but you cant replace them with other geared players, so you get new players. But these new player have crap gear so you have to gear them. This where the store comes in handy and actually IS healthy for the server : its a catchup mechanic. Because what do you think happens when you have to keep going back to old content to gear new players ? Your current roster leave, that's what happens.
Now to get on DST on the DST topic, its not as much that I want it to be on the store, I just feel it needs to be there if healer / tank / caster also have all their gear available. Eye of Quagmire is as sought, rare and BiS as DST, but its on a daily 5 man reset and in the store, whereas DST is on a weekly reset for 25 people, therefore it jusy feels like its not as rare/sought. I just want all class to be equals, not having DST is just hurting melee even more on an expansion that is already caster friendly.