The reward pool will be 1/10th of what it is now
Correct, but incomplete. The reward pool is now about 43000 STEEM/day, in 20 years it will be around 4300 STEEM/day.
Today 1 STEEM is (roughly) $1.5, so the entire reward pool is worth $64.500. But in 20 years, the price of STEEM can be 10x the price of today (in a very conservative prediction) so the future value of the reward pool will be worth 4300 * $15 = $64.500.
The reward pool decreases in STEEM, not in the market value of STEEM, which is something completely different. The same goes for SP.
RE: The Real Inflation Rate Of Steem - Bluepaper Clarifications (Spoiler: It's Better Than I Thought)