Being attached to one of the biggest stars in WWE helps tons with longevity.
Paul Heyman pretty much said that in his interview with The Ringer. He mentions other names that were big stars, but are no longer relevant to today's product. He says being associated with Brock Lesnar is the reason why he's able to maintain relevancy in the WWE:
Tell me somebody who was in the NWA who was relevant then and relevant now. Sting’s not. Lex Luger’s not. It’s Ric Flair and Paul Heyman. Tell me someone from that entire era who is relevant today. Vince McMahon. Jerry Lawler. And of all those people that I’ve named, how many of them are in a prominent position on television today? The only one is me. So, how? The manner in which you were treated behind the scenes did not promote longevity. You were going to be used up and spit out. And here I am as the advocate of the number-one attraction, the highest-paid commodity, the top champion on the flagship show in the company that owns 99 percent of the market share. Obviously whatever I was doing back then, that people perceived as career suicide, gave me the ability to survive long enough to have a longevity that no one else enjoys.
When asked why he only manages Brock Lesnar in the WWE, the answer was simple:
As long as Brock is in WWE. I don’t think it makes sense to work with someone else.
While Heyman has managed others in the past, including CM Punk, Cesaro and Curtis Axel, his partnership with Brock Lesnar has stood the test of time -- since his return to WWE in 2012, Heyman has always been by Lesnar's side.