Let's just say it happened, science in the trash and all. I'd find it funny, and if we followed the same path then all of us would be technically the leader of all monkeys, and the rulers of the animal kingdom. There would be racism of course, and war for supremacy against other monkeys - possibly the less intelligent monkeys seeking to usurp our positions if the stimulus 'we' provide as smarter monkeys strong enough
I mean, if we evolved in a way that our appearances aren't different from normal monkeys. Having hair all over the body, primate biology, noisy voices, and varying physical conditions and faces depending on the evolution. Homosapiens monkey-looking version if you will. Humanity rocks though, but we are also the most invasive species out there in a certain sense.
I am not a biologist, so I am not sure how likely would monkeys developing the glottal stop. but without it, spoken language will be difficult... can replace it with telepathy, or sign lang. I suppose.
but language, in theory, encourage civilization and increase brain cap. to trade that for a tail...hmm interesting. Generally marsupials are smaller that great apes...hmm. so smaller houses...