There are no current plans to make any changes to the Boost algorithm in terms of how long stories are Boosted. Your metaphor that there will be less front seats available assumes that the size of the auditorium is static... but Medium is growing quickly. We just hit a million members in April, and are continuing to grow even more... so there are just more and more readers to serve stories to. We're a loooooong way from having too many stories to share and not enough readers to read them.