What is the opinion on speaking in the colloquial dialect? And would its disappearance be better for Arabs? And when did the colloquial dialect replace Classical Arabic?

There is no harm in speaking in the vernacular with common people; in fact, it is better for them to understand, even if the speaker is proficient in classical Arabic. Speaking in classical Arabic with commoners might be a form of showing off or affectation. However, when addressing students of knowledge and in scholarly gatherings, it is appropriate to speak in classical Arabic.