Off the top of my head, i say... Jorge Sebastien Heiland Eleider Alvarez These two guys have been mandatories for so long and have been taking step aside money continuously which makes me think they're ok with living off step aside money and never going for their title shot. Alvarez has been mandatory to Stevenson since 2015 and has been chilling in Montreal with step aside money ever since. Never forcing Stevenson to take the fight. Ditto with Heiland, he's been GGG's mando for a while and just chilling in Argentina. I honestly think these guys are cool with their mando position and cashing in those step aside money cheques.
I think you have to add GGG to that list. Along with some corruption by the WBC they allowed Cotto and Canelo to fight Geale and Khan and paying GGG step aside money when all along his camp was happy to accept step aside money. I believe Cotto didn't even pay the step aside money GGG was owed when he agreed to defend the belt against Canelo. He was also taking step aside money from Sturm. Luis Ortiz can be added to that list as well.
Any knowledge as to how many times they have been paid? I imagine Alvarez and Heiland never got any real money inside the ring, and those step aside fees might be career high paydays(or close to it).