The pool will never go dry. There will always be STEEM created. The inflation rate does keep declining over a 20 year period until it reaches under 1%. From there, it is a steady issue each year.
Also, SBD do not have a limit, per se...although there is a point built in where the amount of payout in SBD is reduced (as we are seeing now) and, ultimately, where they are not produced. This all ties into the amount of SBD as a percent of the STEEM outstanding.
So... What's the reason some people say that the pool is going to get dry?
I am not sure what you are referring to exactly.
One thing that is becoming evident is that it is going to dry up for bloggers and other individuals posters. That is where the SMT will come in. With the developers and entrepreneurs starting to enter, they are the ones who are going to start amassing SP.
@steemhunt got a delegation of almost 1.5M SP. That is a lot in their hands. They are going to take a larger portion of the reward pool since all those people who delegated no longer have that VP at their disposal.