The seen and be seen principle was designed at a time where a/c were few and slow,it doesn't work well when one (or both ) is doing 300 Kts
But it is the best we have still .
Mixing IFR jets with slow VFR pistons in same piece of airspace often on différent frequencies is the issue , and as their number is growing the rate of such collision will be too.
The current technological solutions are either too expensive for GA , or not really working (e.g ADS-B) Segregation is another solution but extremely unpopular.
But if the number of death raise to an unacceptable level , that is probably what will happen in the end .