As block rewards lessen, the inflation of Bitcoin nears zero. Deflation, in monetary supply terms and disregarding lost coins, is never a reality for Bitcoin. However, as you have already noted, price deflation is possible because of demand outpacing supply.
The number of bitcoins expected to enter circulation is predictable and expected. This means that inflation is already largely factored into the price. Therefore, even with decreasing block rewards, we should not expect significant appreciations in price if demand is unchanged. [Edit: However, I am not 100% sure how swift changes in monetary creation affects velocity/prices, as will happen at each halving] Of course, in this scenario we are assuming no surprises, like a large sum of coins being verifiably lost.
If ten times more people are using Bitcoin in 2020 as now, this does not necessarily mean that demand is 10x higher. Demand is not a number, but a schedule. See the following for an in depth explanation:
http://mises.org/rothbard/mes/chap11a.aspHowever, because demand is not simply a number, this also means that the price can increase by a factor greater than its adoption base. In addition to the number of users, increased demand might result from improved functionality, infrastructure, velocity, etc.
As an aside, I think there will be shocks to come if very old coins begin to move. As time goes on, the market is factoring old coins out of the price, but if they begin moving, they will be part of the money supply again.
