Downey WAS a major star up until the mid 90s, when he went to jail for drug possession and some other things...

it wasn't until he got out and popped up on Ally MacBeal that his career slowly started to turn around...

once Downey did Kiss Kiss Bang Bang in 2005 then did Iron Man a few years later..his career TRULY TOOK OFF (rightfully so) and catapulted him BACK onto the A list, some 15+ years later...

Jude Law WAS A-list from jump, but a HOST of bombs and a messy divorce and personal life threw him OFF...and he's now trying to get A list status now having to compete with the likes of Christian Bale, Sam Worthington, and newcomers like Tom Hardy...

so, in theory, Law SHOULD have been Sherlock but Downey IS the perfect fit in the role.