Why Bismarck used a 15-inch main battery

Was it inevitable that Bismarck would receive 15-inch guns?

They were the readily viable industrial choice, but larger foreign guns would create pressure to go bigger at the cost of time and effort.

Fifteen-inch guns were the viable option the existing infrastructure could deliver at the time. That does not make the choice inevitable under different foreign threats. If 16-inch or 16.5-inch batteries were standard abroad, Germany would probably try to exceed 15 inches. Developing that larger gun would take longer and impose its own tradeoffs.