The guy is superhuman. If he wanted to, he could probably get a 6th at age 35. Anthony Ervin is doing it at that age in the 100 free.
Phelps might be the most dominant athlete in the history of sports. Jordan, Gretzky, Ali... guys like that are more well known and revered because their sports are (were) more popular but consider this. Although those guys are widely considered the best of all time in their sports, you can typically find people who are going to make a case for someone else. Chamberlain, Lemieux, Marciano, Louis, etc. I don't know if it's even possible to make a case for anyone but Phelps as the greatest swimmer ever. There's really no logical argument against him. He owns every record there is for both peak performance and longevity and while the former records will be broken someday as humans continue to evolve, the latter may never be. I'd be stupefied to see anyone sniff 20 gold medals again in my lifetime.