That was Q's assumption in the heat of a crisis when he saw Silva's escape plan on the laptop.
That was Q explaining it for us. Because it was such a silly plan.
And if Silva was locked inside a normal cell (and not that space aged fish tank) I assume dozens of his men would've stormed the MI6 bunker instead to get him out.
It's interesting... for this particular plan (if we go with the idea he had more than one...which he didn't) he HAD to be locked in the fish tank...
But, let's go with your idea... that he was locked in a normal cell... how would his men know? Are they sitting around waiting for a phone call? "Hey, it's me. Silva. Yeah. We're not going with the fish tank plan... Yeah. They put me in a normal cell. Yeah. I know. I had that computer virus all set to go... Sigh. Anyway... get your men and try to attack..."
The harsh reality: this was a poorly plotted film and you can justify it all you want, but it requires making up stuff that's not in the film.
And if Bond was vitally important to Silva in his mess of a plan how come Silva carried on with his plan to undermine the Secret Service and M after Bond was pretty much confirmed dead by M (and also informed by his henchman who stole the hard drive McGuffin and grappled with Bond)?
But Bond ISN'T important to Silva's plan, he's only important to the film. Any agent could've taken in Silva--but of course, this is a Bond film, so it has to be Bond. But, Bond basically helped Silva all the way through. Which makes one wonder, maybe Bond was working with Silva...?
And, while we are making things up... if Silva has multiple plans to humiliate M, why did they not end up with Silva just breaking into her house after things had gone all wonky for MI6? Easy peasy. Even an agent without equipment could do it. You think a hacker like Silva couldn't?
It would have been FAR more easier for him to have bread crumbs that lead to someone in the South Pacific, or where ever they were, while he was sitting in Britain, watching the BBC report who MI6 is falling apart and that its Ms fault, then take a cab over one night, break in, and shoot her. Or blow her up. Or poison her. Or choke her. Whatever. While agents are looking for him on the other side of the planet... I mean, if we're speaking of made up multiple plans.