What's your supporting argument for thinking they would not be as good?
We can point to the Bills breaking the playoff drought with Tyrod Taylor and Co.
I think "the process" is there for the Bills to be a playoff caliber (not championship caliber) team with any above-average QB under center.
For Bills fans focusing solely on the Bills, I think perspective can be skewed a bit. For a few years now, the Bills have had one of the better overall rosters in the league. With Cousins playing QB, they'd probably still win the AFCE, but at least be a WC.