It was not "debunked" in any way. We know 100% they colluded with Russian agents. They just couldn't prove criminal intent. As far as Trump's obstruction of justice, correct me if I'm misremembering, but I'm pretty sure the Mueller report basically said "He did it, but we couldn't come up with solid enough evidence that we would be confident we could indict a sitting president" and the last line saying something like "this absolutely does not absolve the president of wrongdoing". Basically if Trump was anyone other than the president, they would have charged him.

Without 100% rock-solid evidence, the legal complexity of trying to indict him (and him potentially pardoning himself) would have likely caused a constitutional crisis, and who knows what might have happened then. It wasn't worth the risk.