Kylie Minogue's last single was her least successful. Only reached 96.
Madonna's last single also failed to make the top 40:
You can see how hard it is for older artists to have chart singles success. Times change! Teenagers aren't going to relate to artists like Madonna, Kylie and Cher.
That wasn't Madonna's last single - that was Turn Up The Radio, which reached something like #173. Her poorest performer before that was I believe Revolver (from the Celebration Greatest Hits) which was also number 100-something. She also had a 100-something with 'Hey You' which I believe was download-only.
I love Madonna and I'm so sad her singles success has faltered laterally. I don't disagree with your point though. I'm under no illusion though that it's down to any mystery forces other than airplay/videoplay and the commerciality of being a late 40s/early 50s something female 'dance' music singer (Hey You aside, being an acoustic ballad but only ever released to tie in with Live Earth or whatever concert it was) - especially when you consider that the music style itself is not so different from anything riding high in the charts at the time. Most singles are bought by teens and of course they're not going to be interested - she's had her time. The time comes to all 'famous' acts that they just stop having hits, it doesn't stop them being famous or indeed 'successful' in broader musical terms. That's not to say 'flukes' can't happen, like someone touched on earlier with Cher and Believe - although again, in the download as opposed to physical single age I wonder if that would have been a different story today.