It's really really difficult, but possible. Thing is, you need to be either a top sprinter, or an insane descender + amazingly strong on the flat to win MSR. Then you need to be really really punchy to win LBL, great at cobbles for PR, and then a combination of both, ideally with a good sprint, for RVV. Finding a rider like that is next to impossible, even over the course of a career. If Cancellara was a bit younger, then maybe he after losing weight, he would stand a chance in some of the ardennes. Degenkolb, Sagan and Thomas are all also potentially capable, but really, the closest rider to fitting that description, imo, is Van Avermaet - punchy, good on the cobbles and a fast finish, but at the age of 30, he's probably too old to do it now, along with Cancellara and Thomas
Looking at some younger riders, Alaphilippe looks like he could potentially do it, but really, it's Benoot who stands out for me. He seems like a punchy rider who is good on the cobbles, and with the right training, could win all 5 over the course of his career.