User blog comment:SolitaryReaper/Shepard in Javik's position/@comment-5267262-20120726225450/@comment-1221773-20120727060104

I believe Sovereign served that purpose. He'd judge if the galaxy was ready for Reaping and remotely activate the Citadel relay. If the Reapers could remove the Protheans' alterations to the keepers, they would probably pick another vanguard.