If someone got extremely constipated & got intestinal obstruction, wouldn't they have fecal vomiting or something 2 where they'd no something's wrong?

Not necessarily : Our bodies are amazing in terms of what we can get use to. Some can go upto 2 weeks without bm! most people will have increasing abdominal distention and some can have vomiting. Also lots of pain, so yes people know something is not right.

Yes: Bowel obstruction can result from unmoved stool blocking up the colon. Bowel obstuctions, regardless of cause, lead to vomiting, feculant vomiting, dehydration, electrolyte disturbances and need for emergent care/surgery. Avoiding such a dire consequence with a bowel regimen and two soft bowel movements daily is much better for your health.