Personally I think if Monero overtakes Dash it will be more likely because of panic dumping that happens very quickly than a gradual technology "run into wall" slower failure.
The start of the panic dump could be some kind of news article that shows Dash's flaws, some kind of private send privacy breach, or a master-node dump.
Of course it could be a slow grind, or maybe Monero will die before Dash, my crystal ball is often on the fritz (I brought Slater and Gordon shares after all....)