in fact outland wasnt grindy enough.
TBC is by it's very nature grindy that is what most die hard tbc players like about the expansion.
The introduction of bis items like glaives to the shop didnt sit well with a lot of the top guilds which is why a lot of people quit u could see the population go down from about 6-7k players online at average to ~4k in the span of a month after this was implemented.
Another reason Outland died is simply because tbc has not nearly as much potential as wotlk in terms of holding players, most people play tbc for the progression through the content which at some point is simply finished there is only so much you can reach in tbc and a lot of the players that quit were done after farming swp for months and reaching their goals in arenas especially given the fact that farming bis items was very much devalued anyway as everything but thoridal became available through the shop, even the season 4 shoulders which for most tbc players is like a symbol of status and skill due to the high requirement that you have to meet in order to obtain them, putting such items in the shop for a few bucks hit the pvp community which broke apart after the initial season 4 release just as an example.
Then there is the fact that tbc by nature divides it's playerbase between the people that go tryhard and get access to all the content and those that do not as the latest stage of raiding is very hard and there are no difficulty levels as in wrath to give the more casual players a chance to see the latest content aswell, basically meaning they're locked out from it to begin with unless they're willing to spend excessive amounts of money and/or time in order to meet the required gear.
Imo the best options that WM Staff could do to revive this project would for once include a character wipe aswell as a restructure of the very server concept which can go different ways like either to make it a pvp only project much like Blackrock or formerly Arena-Tournament or to lean towards a more retail tbc like experience, either way the timing is not ideal right now to take steps as it would make more sense to wait for the classic hype to die down a bit to appeal to a larger playerbase.
They stated about 8 or 9 months ago that they got plans for Outland so I expect something will be happening at some point in the future.