So when Smackdown went to Fox, it went from around 2 million viewers on USA to 2.4 million viewers once the first few weeks cooled down to normal levels (and before covid). So like a 20% increase.
So this 40+% decrease year-over-year is way more than just the switch back to cable. It shows a clear lack of interest in Smackdown compared to last year. This week the decrease is bigger than usual because of The Rock last year, but look, last year was around 2.4m almost all the way to Mania
https://www.itnwwe.com/wrestling/wwe-smackdown-rating-viewership/, so it's still close to 40% each week.