Would Rihanna have been MJ, Mariah, Whitney, Elvis, Stevie Nicks, The Beatles, Madonna, etc level of fame…

If she, well first... she came out in 2005 and didn't become big until 2007. If she was around during the late 90s to mid 2000s were pop was still significantly bigger, you still had less opportunities to become famous, thus less famous people and each individual one has more fame, and sales/streams/charts were still a major key factor, as due to all the streaming platforms and services, and there just being too many ways of listening to music, the only major key factor now is impact/rememberance, and during the late 90s to mid 2000s you had the rise in the internet, CDs were more easily accessible, and you still lacked a streaming era making it harder to find acts outside of the mainstream.