I don't see how switching is any more difficult now than previously
I'm guessing that in the "old days" in BA when other dances were played, the people who danced the other dances did not have to learn them in a totally separate context and environment from learning their tango the way we do (at least here in the US). There was also less variety of tango within their own tango community, so the other dances provided variety.
Now you would likely be learning tango in a completely different studio with different people and teachers who don't cross over much. If you do go to a ballroom studio to learn AT, you run the risk of getting taught by a teacher who actually doesn't know a lick about AT, but has watched some videos and Dancing with the Stars.
So it wouldn't seem out of the realm of possibility that switching is harder now, because the learning experiences would be so separated.
And also, as I said, I think (at least here in my area) we get such a wide variety of music and styles even withing the "tango" genre at a milonga (without even considering vals and milonga) that the "other dances" are not needed for variety the way they might have been at a crowded traditional salon in BA in the past.