It will not be the death knell for Rochdale. Just the opposite, it will stabilize and strengthen them.
It will be the death of them though as an independent club with their own ambitions. They will be beholden to Saints for their players, coaches by the looks of it, and, should they wish to divorce from the agreement with Saints they will not have the infrastructure left to sustain them as an independent team or club. They will be a feeder club with all that this implies, a nursery for Saints juniors. This might be all Rochdale are ever capable of being, a Championship, Championship 1 team and, if so, it could be good for them.
If they have ambitions for the top flight though, those are gone foreever.
Thats actually a decent summary, but they won't be a full on feeder club as they can only play up to 5 Saints players at a time, and thats only if they're good enough to get into the team.
Ambitions for SuperLeague can still be there if they ever got themselves a nice sugar daddy, but until that happens this is a very good alternative. The exact same goes for my club York with Hull FC.