Whitney is turning over in her grave right now over this discussion.
Of course she is, she is probably dancing to one of Rihanna's singles liek the rest of the world. Do you think its "Where Have you been" or something more R&B like "loveeeeeee song" or even "We found love" tbh.
I think it's kind of unfair that you cite Madonna's last album failing as a sign of her not being iconic. She's been in the music industry for thirty years. Katy Perry has only been around for five. I highly doubt she'll be relevant in 2038. Younger and newer artists always sell more than their older counterparts.
Also, Madonna became iconic due to being one of the most controversial pop stars of all time. Almost all of her eras have media controversy attached to it. When was the last time Katy Perry did anything outlandish or out of the norm? The only thing that comes to mind is the sesame street incident.