[cies]

I'd say, more specifically, it has to be a lot more modular.

[gmueckl]

Modular in which sense? Clean interfaces between non-exchangeable modules or the actual ability to replace modules?

[marcosdumay]

Clean interfaces imply on exchangeable modules.

[gmueckl]

Mo, they don't. They are merely a prerequisite. It must be reasonably possible to reimplement the module in question and it must be possible to instantiate that alternative implementation instead of the original one.

A counter-example is to have all lodules linked into a single static binary. You may have a beautiful separation berween modules in there, but the static linking makes them an immutable bundle.

[cies]

Thus "clean" interfaces make exchangeable modules possible. We still have to write them.