[colordrops]

Not sure what you mean. Quantum effects don't "disappear". They are physical laws. Which slit the particle enters after the wave decoheres is precisely a non-deterministic element of quantum mechanics.

[bouncycastle]

But wouldn't measuring something also alter the outcome? Ie. Observer Effect https://en.m.wikipedia.org/wiki/Observer_effect_(physics)

[colordrops]

It doesn't alter the outcome. It causes it to decohere into a particular outcome. There is no way to force a particular outcome, hence it being non-deterministic.