You're right, you can't have unlimited depth. But there's a pretty wide gap between "unlimited depth" and "three starters followed immediately by practice squad-caliber players". In other words, the inability to have unlimited depth isn't an excuse to have no depth.
In your own evaluation, you accounted for all 11 positions having a backup. Well, who was the backup for Mayo, Collins, and Hightower? It was Deontae Skinner, an UDFA rookie who we signed off of our practice squad and immediately became a starter. There's your problem. I'm not suggesting that they should've had multiple backups all waiting in the wings in case Jerod Mayo specifically got hurt. I'm suggesting that they should've had at least one depth linebacker on the roster who had played a meaningful snap in the NFL. Instead, the Patriots chose to construct a roster where an injury to any one of their three starting LBs meant they would have to start a practice squad player who had never played a meaningful NFL snap. Those three linebacker positions are nearly a third of the starting defense, and the Pats didn't have a game-tested backup for any of them. It was incredibly easy to predict that that would backfire at some point, but even I figured that the Pats would have all three starting LBs healthy for more than the one game that they ended up getting.
As for the other part: who would I have signed? Well, technically they didn't have to make any additional signings. They could've simply not cut James Anderson. Cutting him was what moved the needle, for me, from "they're taking an understandable calculated risk" to "this makes no sense". Another alternative would be signing Woodyard this past offseason, although for the money that he ended up getting I can understand why they didn't. Neither Woodyard nor Anderson would've replaced Mayo, but they would have created some roster flexibility to move players around so that you aren't stuck relying on someone like Deontae Skinner to be a capable starting NFL LB right away.
And Skinner may end up being good. I'm rooting for it, certainly, and it's not like it would be unprecedented. Maybe he ends up being a revelation for us on par with Danny Trevathan for the Broncos last year. But it's foolish to bank on that at three different positions without any alternative to hedge against the likely outcome that he sucks. There's value in having young guys with the potential to grow, but there's also value in having guys who have proven that they can play, and there's no reason not to have both elements at work in your LB corps. This is the 2011 safety rotation all over again.
