Some posters seem to be less than clear about the fact that VMO and MMO are certified limits.
The aircraft does not promptly come apart, but you start to compromise the "advertised" life of the airframe, when you exceed those limits.
To establish those limits, the aircraft has been flown to well beyond those limits in a carefully controlled manner. Concorde has an MMO of 2.04, but the certification aircraft went up to M 2.23. For those who know the Concorde story: it was one of the reasons the two cerification aircraft (201 and 202) never went into service in the end. It was extremely difficult to assess how much of the aircraft's "life" (in terms of fatigue cycles) had been used up by the certification flights to the edges of the envelope.