Slides were definitely for more serious photographers many of whom mail ordered film and sent it off to Kodak or someone else for processing--though I'm sure it was still a distinct minority of film sold. One of my rituals when taking a vacation where I'd be doing a lot of photography was to place an order with B&H (via phone) for 20 or however many rolls of Ektachrome and Kodachrome.

I actually wrote an article for CNET when Kodachrome processing was eventually shut down. https://www.cnet.com/tech/tech-industry/countdown-to-kodachr...