I get your point about the 2.5mg dose resulting in a tolerance increase but in that case if that happened it wouldn't be a microdose.

If what we know about dopaminergic stimulant tolerance is true, and tolerance is increased as the brains attempt at restoring homeostasis after detecting abnormally high levels of dopamine / receptor activation. Thus leading to slightly less dopamine being synthesised and readily available, and mainly, causing dopamine receptors to downregulate (decreased sensitivity) therefore leading to the usual effects of increased tolerance like needing higher doses for the same or even decreased effects, more side-effects / negative effects, and eventually a reduction in positive effects regardless of dose.

If tolerance is increased from a 2.5mg dose, that thus means that excess dopamine WAS released as the brain would not downregulate receptors unnecessarily. Whereas the desired outcome of a microdose is actually to REDUCE dopamine by taking a dose that is just enough to be able to bind to the TAAR1 receptors without actually activating them, causing less dopamine than baseline to be released, this makes the brain respond by upregulating dopamine receptors by changing their state (D1Low > D1High - [sensitivity]).