I don't get the challenging themselves bit personally. I don't really want them to change their styles just to move with the times. I like Floyd for their own unique sound, if they wanted to keep spinning that forever then that suits me fine. I don't want them changing it just because some critic who may or may not even have any musical abilities or training himself thinks they should mix it up.
It is essential, absolutely essential for any artist to listen to the latest music, read the newest book from newer authors, or enjoy sculptures or paintings from the latest painters. You cannot, no matter how talented, continue at a high level if you do not do this. the creative mind requires stimulation.
For example, Waters' biggest influence was Dylan. Specifically, Sad Eye Lady of the Lowlands blew his mind and influenced him into thinking that really long songs were okay as long as they continued to enthrall. But it doesn't just stop at Dylan. It has to keep going. Maybe Waters or Gilmour hears an underground ambient artist they love and nik a little bit, or are inspired to try something new within the context of the Pink floyd sound.
It's not about Pink Floyd totally changing their sound, it's about Pink Floyd as H.P. Lovecraft's Shoggoth, absorbing everything it touches and becoming even more potent and amazing.
It applies to music, writing, art, whatever. Show me a writer who doesn't read other writers on a regular basis, and I'll show you a bad writer. Show me an older rock act that doesn't listen to newer bands, and I'll show you an album that is dull and full of late 80's early 90's studio production.
You can't just own 100 CD's of classic rock acts and expect to be an educated music listener. Nor can you be an educated musician if you do the same thing.