In general, the basic premise--that you need to cast a big star as the main villain in a Trek movie--doesn't really hold up to close examination. Alice Krige had been around forever, but she was hardly a household name when she played the Borg Queen. And what about the guy who played Sybok in TFF? Or even Christopher Lloyd in TSFS. He'd been a supporting character on TAXI, sure, but he wasn't exactly an A-list movie star at the time.
Cumberbatch is the star of a hugely popular British mystery series, which arguably gives him a higher profile than Weller, whose leading man days are at least a decade in a past . . . .
(No disrespect to Weller intended. I'm just pointing out that he's not exactly a big marquee name either.)