Well, till we wake up Durin's Bane, at which point we're #%$!
Now joking aside..


This is pure speculation, daydreaming, being totally positive ...

I think that at one point the big business in crypto will have no choice but to take care of a part of the burden involved in securing the chain.
When you're doing millions in trading, you're having thousands of users with your bitcoin card, you're getting millions in bets the last thing you want is this all of this to go down forever, not just the price going down as business will continue even with BTC at 1k, think Silkroad for example, but completely down with the chain unsecured and 51% making a mess of it and faith in cryptos fucked forever.

So, once there will be signs of dangers the big guys in this will have to protect their business.
Just like exxon is planting trees to calm the environmental activists or politicians like to "donate" things during holidays they will come with so-called nonprofits projects for the sake of decentralization, green energy mining and stuff like this sponsored by them as a great service they are willing to do for the community but strictly financially motivated which is the best motivation you can get.

This combined with also the dream of returning home miners might secure the same hash rate we're seeing now for a long period.
Oh, and don't forget the 21Inc toaster miners

But mining as a large business with tons of profits in 20-30 years, I don't see that possible!