Consider if this was 2013/2017 and we knew that the upcoming expansion was going to be set in alternate draenor, kul tiras and zandalar, and we'd be fighting against, literal, army of technology-users. We'd have literal bosses with mechanical themes and abilities.
And to top it all off, the expansion starts with a military battle, using technological devices because Garrosh took goblins plans to alternate draenor (hey, there's a reason for a Tinker class right there).
Would anyone seriously say that a Tinker class wouldn't fit such an expansion? If a Tinker doesn't fit an expansion like that, one has to truly ask what expansion would a Tinker fit into?
While we're at it, you can add Alchemists to that question as well. I simply don't see a better set up, given the events of WoD and BfA, an opening for Alchemists given explosives themes of the expansions, and the contribution of war science to those expansions. Again, if Alchemists aren't fit to be introduced in those expansions, when would they be?