Solly D got out of prison a few years back. He & Jimmy Inendino are more or less interchangeable, they're both of similar age, they both have done similar amounts of prison time, and they were both eligible for huge promotions due to said prison time. Either could be boss. Probably Jimmy I, but Solly D could be as well. It only makes sense that either would be boss at this point in time. The Outfit tends to reward guys who have done substantial time standing on their heads.