Chiming in late, from what I recall reading, there is an island bus loop planned for Cummer/Drewry for both routes to service west of Yonge. So, I think they may remain separate... likely interline, with some exceptions due to the differences in ridership. But yes, anything for Finch to access would be by subway or Yonge bus at that point.
Plus the Finch buses or Finch LRT.
Btw, Drewry bus route is very short, only about 3 km each way from Yonge & Drewry. Full interlining will be OK, even if the demand on the Drewry section is a bit lower than east of Yonge.