I'd be very surprised if the defense took a major step back.
I won't say there shouldn't be skepticism from transitioning from Golden to Ash, but at the end of the day, there shouldn't be any mistaking that Marcus Freeman has a huge hand in how the defense operates. Unless Ash is BVG-esque in terms of disconnect with the players, I don't see there being a big drop-off, if any at all.
It'd be tough to replace guys like Mills and Cross, but the reality is that we already essentially had to replace them. Mills missed basically the entire CFP, and ND had to make due without him and, for as much as I loved Cross as a player, he was nowhere near the same guy last year, because he was consistently hurt. Dude would absolutely bust his ass every second he played, but you could tell that he was struggling to just walk straight, much less hold up against a CFP level offensive line.
I don't think they can replace the 100% healthy iteration of Cross and Mills, but ND made it all the way to the title game without that and frankly, less than what they have now. If Ash and Freeman can come close to replicating the impact that Golden made, they should be more than fine.