Do you still get random world-spawn dragons after you kill Alduin?
Based on experience1, killing Alduin doesn't kill all of the dragons, so you will still find dragons roaming the province of Skyrim on your next adventure.
The goal of the game wasn't to just get rid of ALL2 of the dragons. It was to only kill Alduin.
Killing Alduin only ends the Main Questline.
1The results might not work with everyone else.
2There are some other certain dragons besides Alduin that you have to kill in order to advance in the Main Questline.