The question makes sense only in the context of what has happened at other airlines, i.e., replacing mainline DC-9s, 737s, etc. with CRJs and ERJs operated by regional affiliates. In some cases, older mainline aircraft were retired and replaced entirely with regional jets.
However, it does not apply to JetBlue for two reasons. First, the Embraers are not replacing any A320s, since the A320 fleet will continue to expand simultaneously. Even if A320s disappear from a particular route, they will simply be moved elsewhere--so, at the end of the day, no "A320 flying" is being "taken away." Second, the Embraers (which really shouldn''t even be lumped in the same category as existing ERJs and CRJs anyway) are being flown by JetBlue pilots, not by a subsidiary or affiliated carrier.
I don''t work for JetBlue, or any other airline (heck, I haven''t even flown on JetBlue), but I can only imagine that it must get tiresome for JetBlue employees to keep hearing this sort of groundless implication of future labor unrest, skyrocketing maintenance costs, or whatever the ominous Chicken Little prediction du jour happens to be. Sure, JetBlue has plenty of challenges ahead of it, but in the meantime, it seems to be doing everything right.