Why exactly does it lean, again? Isn’t it because the earth it’s built on is unstable and making it sink into the ground on one side at a slow rate per year?
And what will eventually become of it? If it’s still sinking, even at a very slow rate, where will it be, say, 100 years from now? Wouldn’t it fall over finally someday?
Many attempts have been made to halt the progress of its lean or right it (famously, the first attempt was carried out while it was still being constructed) - including massive lead weights on a concrete collar and freezing the surrounding soil.
The latest attempt appears to have worked; they drilled out some cores of soil from the side that wasn’t sinking so fast and this allowed it to settle back to a slightly more upright position, albeit slightly lower compared to ground level. Supposedly good for a couple of hundred years now.
Yes, the soft soil has caused the differential settlement of the tower. A few years ago, a team of international engineers completed a “straightening” of the tower - not looking for perfection, but trying to move it several inches.
You may find thisarticle helpful.
The construction of the tower went on for two hundred years. Over that period, several attempts were made to straighten it as it went up - including the use of tapered blocks.
According to CNN, it reopened in December 2001.
Sounds like this “straightening” technique can be used successfully again, if needed.
Upon preview: beat me to it, Mangetout 
Maybe a trite answer, but I suppose that the tower can be kept upright for as long as the somebody thinks it’s worth the cost of doing so. There’s really no reason why the lean cannot be remedied indefinitely, although probably through increasingly more expensive engineering and technological fixes. If it ever reaches the point that the costs of maintaining verticality outweighes the perceived benefits to national pride, historical preservation and tourist dollars, then that’ll be that.
Honestly, I think it’s amazing that the thing is still standing. It REALLY leans. It’s kind of disorienting when you walk up it.