This is a lot of money, I do not know how he can be so optimistic knowing that there are few countries that use bitcoin as a means of payment or even that its population has easy access to be able to buy bitcoin and that will not change in 15 years. So this prediction of him is too exaggerated.
His main incentive is to make sure that the value of his massive stash of coins goes up even further throughout the years. His Bitcoin holdings are worth a few hundred million dollars at today's prices.
If I was in his place I would be shilling the crap out of Bitcoin as well. We will find out what's left of his bullish nature when Bitcoin ends up hitting his targets. Will he cash out entirely or actually use it as a form of money.
Litecoin founder Charlie Lee found it necessary to dump his holdings at peak levels, which might happen with more of these high level promoters of crypto in general. It wouldn't surprise me if T.D. follows.