I don't think it applies only to millennials, even restaurants i've visited recently have a bunch of plants now!
Honestly, I really like this new trend :)
Uhhhh... I had 30 plants in my room alone at the age of 15. Most any of the houseplant books you find were written in the '70s or '80s. What is new is the idea that OMFG millennials are the first generation to keep something potted in their goddamn foyer.