To me, neither the equipment nor the film define "serious" photography; rather it is the intent of the photographer and the images produced.
In 2006 I was in Yosemite and had my SX-70 with me. I made a photo of El Capitan and of Half Dome that impressed me with the creaminess and tone of the colors - right then I knew I had two keepers (which I later scanned and enlarged to 8x10). These two are among my very favorite photos that I've made over the last 30 years with various formats and media. I suppose if I'd used 35mm, or whatever - even digital, I might be able to tweak the results to look similar -- but I knew the shots had an amazing look right after I took them.