Times may be hard in the future, and then change. If you're not here then you won't be able to help participate in what comes next.

It's my belief that pain serves a purpose. What is it? It disrupts things -- actions, intentions, relationships -- that aren't/can't work realistically in the world (any more). Those are "formed" with some joy or anticipation. When someone dies, or we fail at what we intended, or we stub our toe -- that which we are engaged in has to change, too, perhaps, in order to be in sync with what else is going on in the world. And the negativity or pain has a function in that happening inside of us.

It may be that you will be one of those who don't get to weather the next storm or whatever -- and it might be horrible drowning or being poverty-stricken or whatever. But if you can work on things now, and improve your health and mental heath, if you can, then. . .seems to me like your chances are better to find happiness in the future, too. Though it may look very different from what makes you happy now. And it might not work out that way either.

There's really just no way to know or control a lot of it. What can you control? Mindfulness does have some techniques for living calmly in the present. Living fully in the present, like you said you would do if you knew with a 99% certainty that Alzheimer's disease was in your future in 2 years, seems like a good idea, too, to the extent that it is practical -- whether disaster will strike or not.