Your logic implies that any loss can be overcome with zero drop in productivity by just going to other things. Lose Gronk? No big deal!
I haven't seen many people argue that. Everyone in this thread that I've seen seems to agree that Brady and Gronk are the only two truly irreplaceable players. (N.B. They won 11 games without Brady and got to back-to-back AFC Championships without Gronk for large portions)
It's not "no big deal" to lose good players, but this is a team that is always built to weather injuries.
Even since last year they lost Revis and Browner and have been fine on defense, and they lost Vereen, Ridley, Solder, Connolly, Wendell and have been even better on offense.
The Belichick-Brady system works because they manage to replace players that seem irreplaceable. Welker leaves and Edelman replaces him. Mayo gets hurt and Hightower and Collins turn out to be incredible. Stork goes down and Andrews turns out to be fantastic.
I guess I just trust that Belichick, Caserio, McDaniels and the rest know what they're doing as well as anyone has ever known anything, and I trust they were prepared to be good in 2015 even before Lewis turned out to be a diamond in the training camp rough.
One way or another, the team will be fine.