"10-15%" seems to have been standard for nearly a half century, then -- if it was standard a couple of decades after the 1920s through to when I was a kid in the 1980s. That's a lot slower than once a generation.

15% is still an acceptable tip anywhere in the US, a few odd folks online and a few irate servers trying to bully people aside. IOW, I wouldn't worry about it; at the level of changing from a flat 10% to a range of 15-20% in 70 years (and then being stable for another 20) doesn't make it sound like it's individuals "bidding" it up so much as major structural changes in the economy ... and even then, that stuff seems to change faster.