Maintenance is not the issue. Eventually parts fail. And when they are large parts that can't be sourced from another copy of the ride, the company has long since gone under and their designs and IP were never sold to another the ONLY way to replace them is with expensive custom-machined parts that have to be copied from the originals.
Up until its recent rash of failures Xcalibur ran significantly more reliably in SFStL than it did the last few years in Great Adventure. I'm not about to claim that this is because STL is better at maintenance - it likely received a full refurb during the move. And back then they probably actually could cannibalize its recently deceased sibling(s).
Silverwood isn't keeping Aftershock running all on their own. They worked closely with Vekoma when they moved and installed it - making sure to incorporate anything new Vekoma had to say to improve the install and operations of the ride. Vekoma has been providing spare parts throughout Silverwood's time with the ride. And they will for the foreseeable future. These are entirely different situations.