It's already happened a few times, not to this extent but there are variants which greatly reduce the vaccine effectiveness.
There is even now a recombinant variant (2 variants re-combined).
The difference is that we have evidence of it already occurring in a minor way while we don't for other viruses.
There is also an even greater threat that nobody has been talking about. Our lack of ability to control this or slow it down is showing a massive vulnerability that other countries like China do not have. China has been able to control and get a handle on this while we haven't. If any country wanted to destroy the west, this is the way to do it without having to even use any effort. We need to get control of the virus ASAP and show everyone we can because in the long game, more viruses will mysteriously come if we don't and world powers will shift.