(1) Why is international migration a bad thing? More foreign migrants probably start businesses than red-blooded Americans, who tend to want a “full time” job. California and NY have been ports of entry for migration for, oh, the last 100 years or so..
(2) Bigger cities have always been rental markets. Take NYC — most housing in Manhattan was BUILT as rentals, and only subdivided into owned apartments at a later date.
(3) There’s quite a bit of CA that isn’t LA, San Diego, or San Francisco.
(4) Renting out rooms via AirBnB is new. Renting out rooms is nothing new. I knew a elderly Austrian baroness in London who owned a gigantic apartment, and always had 10 or 15 foreign East-bloc students passing through. To some extent she needed the money to upkeep the place, but she also felt she was helping them by giving them cheap(er) housing in a convenient place.
People in cities have also rented out single back rooms for ages to help with the rent. Advertising via word of mouth or on paper.
The newer phenomenon is actually BUYING condos or homes to fully use as AirBnB hotels. Whereas renting out rooms doesn’t affect housing supply, or slightly increases it, renting out entire homes/condos/buildings as “hotels” eats away at supply, driving prices up. I can understand attempts to regulate it.