That argument only holds if he provided the
only marketplace where it was possible to buy contraband drugs.
Every single customer could then and can still withdraw cash at an ATM, and then visit down the chain of "guy that knows a guy" until their non-narc bona fides are established, and the psuedoanonymous cash-for-drugs transaction occurs.
He's in prison for life because the War on Drugs principals needed to set up an example as deterrent, because their usual methods of investigation and interdiction were less effective at fighting obfuscated online services, and his was the most popular at the time. The judges went along with it, because they balanced the interests of the government against one guy who is almost definitely a sleazebag, and found that turning the negative PR from the apparent untouchability of Silk Road users into a win by shutting down the system, seizing the Bitcoin, and railroading the operator was fine. Even if they got reversed a few years later, after it was mostly out of the news, that was still an acceptable result. The claim that he tried to hire a hitman for contract murder definitely put a thumb on that balancing test. Without it, you're just tossing someone in prison for starting up a better Craigslist.
From what I know about the case, the prosecution was probably more influenced by politics than justice. If he were market-making for insider trading tips and discreet temporary matchmaking services, instead of drugs, stolen property, prostitution, and murders, he'd still be in business and richer than Croesus. They nailed him with a pseudo-plausible argument and a pressing need to get online black markets out of the mainstream news. It's the same reason Pirate Bay keeps losing domain names, while lesser-known trackers never seem to get the same level of opposition. The first rule of your online black market is to not talk about your online black market. If it gets too popular, it becomes the enforcement target pour encourager les autres.
There's no way in Hell he'd merit a life sentence without the attempted murder claims. Silk Road wasn't intrinsically more criminal than Visa or Mastercard payment clearing services, aside from it not automatically banning merchant accounts suspected of engaging in certain lines of business that are illegal in most jurisdictions. Silk Road was making the War on Drugs actors look like chumps, so they made Dread Pirate Roberts their public enemy number one.
I don't think he deserves the sentence that he got, but I also don't think they're ever going to let him out, because he's back in the news the instant they turn him loose. It'd be nice to believe that the justice system was more about the justice than the system, but I just can't.