CN has plenty of capacity between Bramalea and Mount Pleasant to accommodate hourly GO service at all times as well as their existing freights.
Ten years ago, Brampton Station had one platform, and there was a single-track section between Union Street (east of Brampton GO) and Kennedy Road. That suited CN's needs just fine. Now that has been widened to two tracks, with a third track laid between McMurchy Avenue (west of Brampton GO) and Mount Pleasant, and between Kennedy Road and Bramalea, giving CN the same capacity as before if GO takes the southern track, suitable for hourly service, and CN is being difficult here.
While the missing link is needed for anything beyond hourly service, and the province's plans for high speed rail to Kitchener and London, it's not needed for simple evening and weekend service.