Human brain has a limited ability to juggle a large amount of independent balls at the same time. This is a biological constraint, there is no Moore's law for human brains.
Code complexity is given by the invariants size, not by the actual code size.
If you couple N concerns in 16 lines of code, your invariants may become quite large and you have to keep them all at the same time in your head. Worse, the invariants may dramatically change when you do a "one liner" bug fix. When you decouple your N concerns into N code units, then each unit of code has a clear simple easy to understand meaning/concern. Bonus, there is a lot less risk to have you app subtly do something else as a result of "minor" changes.
I disagree; in fact I come to the exact opposite conclusion based on the human brain's limited cognitive ability.
I believe that when you decouple code the way Backbone (and similar frameworks) would have you do, you may simplify the individual bits, but you've obfuscated how they fit together and interact. In fact, you've increased the number of invariants because you've made it possible for the individual bits to be put together in many different ways, not just the one way that the tightly-coupled code allows. You started with N concerns, and now you have N^2 concerns. That's much worse.
It's much riskier too... when your code is decoupled, how can you possibly tell what effect a minor change will have? It's like programming with nothing but global variables. In the tightly-coupled version, you know exactly every single possible code path through your change, and you can ensure that the new behavior will be correct. You just can't do that in the decoupled version, at least not in anywhere close to the same amount of development time/effort.
Code complexity is given by the invariants size, not by the actual code size.
If you couple N concerns in 16 lines of code, your invariants may become quite large and you have to keep them all at the same time in your head. Worse, the invariants may dramatically change when you do a "one liner" bug fix. When you decouple your N concerns into N code units, then each unit of code has a clear simple easy to understand meaning/concern. Bonus, there is a lot less risk to have you app subtly do something else as a result of "minor" changes.