Where are you hearing this? I keep reading that Montoya is the front-runner for the 78 team right now. I also don't see Ty moving up immediately to the Cup series, since he has yet to run a full season in Nationwide.
So far, Austin spent two years full-time in Trucks (2010-2011) before moving up to NW, where he spent two more years (2012-2013), before moving up to Sprint Cup (2014).
If Ty follows the same developmental path, he still has at least two years to run full-time in NW before he is going to move up to Cup. That might even be a stretch, since Austin has been more successful in the lower series than Ty, so far.