perhaps the solution isn't more but rather more difficult. not nightmare modes but something nonwow clone hard. you can even have content that might reguire special gear abilities only on gear from the previous tier. you must have the gear or you wont be able to survive the encounter. there are several things that can be done when you leave the wow clone encounters mold behind and move on to real difficult content.
The problem with op gating is you shut down the number of players who are available to play. If a guild is on to the next tier of Op and three of their raiders (for example) have various real life issues to deal with, finding replacements is infinitely more difficult and requires the guild to go back and run the older instance and hope for the right drops to get the replacements up to speed.
On the other hand, if you aren't gating on gear you can carry the replacements for a little while until the current content drops catch them up.
Many don't think so, but it really is a good thing that so many people are getting geared up on ops and BH gear. It will benefit the top guilds whether they think so or not (lots of good players who were behind in gear and couldn't necessarily catch up to the point where the top guilds would give them a shot.)