It's foolish to believe WoW will remain a subbed game forever, considering how much money companies make from having their games go free-to-play. Despite that, WoW has a loyal and addicted group of fanbase that will never quit no matter what (probably over 500,000 of people like this) which could sustain subs for another decade.
With the Microsoft acquisition, we could see WoW go free to play even next year, with seasonal bundles and more micotransactions.
I personally believe it will go free to play a year after the microsoft acquisiton (2024), as microsoft will likely have it on gamepass and will see no real reason to keep it subbed.