How long will xbox one digital games available for download?

When you buy the digital copy, you own the game. As long as a service is available to download the game, you would be able to. With Xbox expanding their digital collection through backwards compatibility, I wouldnt expect the loss of a digitally owned title
to become an issue.

Not sure what 2035 looks like though

Well, a lot of Xbox 360 games are still available and downloadable for the 360 so only time will tell honestly.

Who knows what the future holds for gaming

I think that they always be yours in one way or another. They always will be digital. Maybe holographic in future, maybe on some server very small but very powerful, maybe in your on home. Future will tell..

I think it all depends on the direction that Xbox take with the Xbox. Maybe there will come a point where they close the Xbox One digital store, but hopefully not. If they ever do close it, I imagine they would give us some sort of advanced notification
that you can't miss. Should the store close, you will only need to download any digital games that you own, and then you will always be able to play them.