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> All that can be changed as well but at some point it gets complicated!

Not everything has to be changed, that's just a strawman.

But not every detail has to be nailed down at the beginning either.

The idea behind Agile is to reduce risk. If there is no risk of the bus system changing on you, there's no harm in fixing it right at the beginning.

But if there is a chance it might, you'd do well to have systems in place which enable you to react to changes with a minimum (though non-zero, of course) amount of pain.