Aside from the Finch buses, I don't know what was still planned to continue runing into Finch. Neither the current 60, 53 or 42 or 125 would still go there. Obviously none of the YRT/Viva either. Not sure if GO or if the 97 bus would continue. Would be literally hundreds of buses an hour coming off of the stretch between 7 and Finch which, in terms of GHG emissions and road wear-and-tear alone (some of the worst bus knuckles in the region) is one of the least-talked-about benefits of opening the subway.
Can't handle what? They'll still have the terminal (perhaps better integrated with the subway and GO) and Yonge/7 and presumably service along Yonge would operate very, very differently than it does now; maybe no more Viva and just a local bus, like TTC's 97? That stuff is years down the road.