There doesn’t need to be a solution because the problem you’re describing is not a real problem.
If someone quits, they miss out on the current content. When they come back and catch up it’s to participate in current content. It does not matter that they didn’t do the past content to participate in the current content. They missed out on the experience of the current content when it was current and can’t get that back.
For example, when I quit in Naxx in WotLK I did not come back until Trial was about to come out. I caught up in a week getting a full Ulduar set via heroics. Was this unfair to the people who did Ulduar? You think it was, but you’re wrong. It was not. I missed out on Ulduar. I did go back later to get the mount achievements, but not slowly progressing through Ulduar before doing brand new content isn’t “rewarding me for quitting,” it’s enticing people who quit to actually come back.
Catch up mechanics are not to entice you to quit, they are to entice you to come back after you quit by making it seem reasonable to actually play current content rather than after you quit an MMO feeling like there’s no point in going back because you’re hopelessly behind until the next expansion. Catch up mechanics were a solution to a problem that the game had with people quitting and then feeling like there was no point coming back because they were hopelessly behind.