Well they didn't use exclusive contracts but rather forced DRM with steamworks which pretty much killed off physical pc games. I think it was around 2008 when every physical pc game had steamworks DRM and you were forced to install it from steam making physical pointless.I don't remember them as exclusives as such. I do remember that no other company stepped up for electronic distribution. Valve as far as I know has never signed an exclusive contract for games, it goes against the company ethos.
Valve still made some great games back then and had fantastic sales so I had no issues with Steam. Now they don't bother to make games because steam prints money and the sales are worse than console sales. Games you used to be able to get for a pound or two like oblivion or vampire the masquerade will be lucky to go for a fiver these days.