The 6mm was not killed by the magnum craze. It was killed by the 243.
When the 6mm Remington was introduced as a commercial offering, there were three basic action lengths, Magnum, Long and Short. There were four common cartridge cases. These four were based on the the 300 H&H, the 30-06, the 7x57 and the 308 Winchester. The 6mm Remington was based on the 7x57 case.
If you notice, all calibers based on the 57mm case have lost their market since the rise in popularity of the short action designed for 308 length cartridges and light as possible rifles back late 80's or so. The 57mm case is too long for the short action. It can be made to work, but bullets need to be seated deeper which intrudes on case capacity, especially with longer bullets. The 57mm case had plenty of room in long actions but there wasted length. Why chamber a long action for a 7x57 when a 280 Remington has better performance?
6mm Remington, 257 Roberts, 7x57 and 8x57 are all fine calibers. But their 57mm case places them awkward position. Too short for long actions and too long for short actions