Can anyone point me to a reliable source that explains in layman's terms why one cancels out the other and it's not possible to have both (say) delta and omicron at once. Somewhat tongue in cheek, but since cancellation seems to be the case, could we not create a version that was 100x more transmissible than omicron but completely benign? Cane toads in Australia aside ;)
You overestimate our abilities to make viruses that do what we want, let alone to make viruses that do what we want and won't evolve into viruses that do what we don't want after being released into the wild.
This is also basically saying "can't we make a transmissible vaccine", and as much as I like vaccine mandates, I think there would be significant ethical issues with that.