[kstenerud]

SOLID is useful as a rule-of-thumb, but there's always one over-arching principle that should trump all others: YAGNI.

It's easy to build something simple (and perhaps naive) and make it more complex over time as your needs become selectively more complicated and your grasp of the domain more nuanced, but it's impossible to start complex and pare down to a more simple implementation. Even worse, your bad design decisions due to your less-than-complete grasp of the domain become impossible to remove due to all the layers.

There is no One Right Answer (tm) to system architecture; it comes with experience. All of these principles should be considered guidelines to help in your design, not ironclad rules.

[marcosdumay]

Except when you will surely need it, just not now, and will have to change your entire core everything to bolt it on.

People saying YAGNI usually make much worse atrocities than the ones aiming for SOLID. And on SOLID, the I alone may be responsible for the overwhelming majority of the problems.