Here's the thing, Minecraft is going on about 7 years old which is almost a generation in terms of its lifespan. For a game of its genre, there's nothing really holding it up other than the people who play it. Eventually, they will get bored no matter how great of an update they get, everything goes in cycles.
In the past two years alone, we got things like OverWatch, Paladins, and Fallout 4, Nier, Horizon Zero Dawn, and Nioh. Minecraft is a lego box with no real story to the game or any real goals to achieve. In a sense, those playing and creating content are what keep the game from going truly under, not the people making the updates.
What they would have to do to bring in a majority, though, would be to finally give in and release a modding API as well as shoving in the popular mods that many have been crying for for years such as Treecapitator, IndustrialCraft/BuildCraft, and something like Biomes o' Plenty/ExtraBiomesXL. This wouldn't be so bad, but not everyone wants to do technology stuff, hence why there were not added. It's up to the person that wants to customize their experience.
If that isn't the biggest reason, then another I could think of is terrain generation. If you've ever played the older versions, such as the Alphas or anything before Beta 1.7.9, then you would remember the random floating islands, oceans that were only a few blocks deep instead of tens of blocks, and the next continent being within sight of shore.
Clay used to be found above water on beaches, now you have to dig into the water to get them. I could list off a number of ways to improve the game, but sadly it will only be a temporary fix. If you want to keep the game from dying, then you would need to get people to play and create. How do you do that? Simple answer is that you must be the first to the party and last to leave. This means when you get an interest in something, it will spread outward in a domino fashion.