...
If movies were canon, it'd mean that Kaitou KID is aware of Conan's identity, Ran has confessed multiple times, Conan has done every single suspicious action that is possible for even the dullest of people to suspect him, Conan is able to defy gravity and nature (which, IMO, somewhat defeats the purpose of DC being a "mystery" anime, though we can't exactly say it was realistic to start with), Tokyo is going to go into a frenzy over what happened at Touto Tower, the BO will likely be investigated for the case (risking their anonymity), Gin would start digging deep into what happened at Touto Tower (I mean, c'mon. We could all tell from the scope that it was a kid who was climbing up those stairs, wouldn't Gin be able to as well?), Conan's name would be amazingly famous (which in itself would cause suspicion and in-depth research by the BO), Conan would be involved in some stupid pirate plot with Ran and Sonoko that never made sense in the first place and was just overall stupid, and it would be nearly impossible to make it even somewhat make sense. It would throw the entire plot off balance.
While Gosho did put Shiratori into the canon, it's not necessarily a hint to the fact that movies would be canon. It's basically saying, "Someone came up with an interesting character that I can use, so why not?"
If you think about it, if the movies were canon, we'd have a really messed up world of DC. And at least some of us would probably have stopped reading.
...And I just got a plunnie. Perfect timing.