That's unlike the situation we have now where people only raid a couple of weeks until they lose every illusion of ever getting anything out of it and cease to go there.
On a more serious note: We have a lot of people with full T4 toons who still go to the JC. You're right that there are also some who won't go there again once the toon is full but there are enough people of the other kind that keep T4 running. Most have alts they bring in after the main is full, so T4 won't be abandoned. So the issue you're mentioning above isn't a problem in T4.
That means there is something wrong with T3.5.
What T3.5 lacks with these low drop rates is a kind of progression. For me, it was progression to relearn the fights, kill the new bat and get faster clears in the following weeks. That was fun. Now that that we have farm status again with fast kills as well, the only progression left is progression in the equipment of your toons and of the raid as a whole. This is a progression you barely notice, so going there often feels like standing still. I still sign up for T3.5 but I can understand that it's difficult to motivate people to do so as well.