it takes care of it and gives you a glimpse of how things could be.... until it fills back up with other coins peers that it learns from a node it connects to.

if we could get every node to delete peers.dat at the same instant, it would last longer, or forever.

This is where we may be able to improve the situation if we deprecate old versions.. and maybe in conjunction don't load addresses from some "less old" versions.

I'm running .9 with only the single new magic num in it, peers.dat fills up same as always.

The problem as I understand it is that peer addresses are exchanged pretty much at the begining of any new connection. If you delete peers.dat, you have now made all subsequent connections "new" since there is no history of last contact with any address. so your client seeds from IRC or dnsseed addnode= etc and rx's a peer list from each new connection. So unless each of those connections has deleted his peers.dat and started fresh, one or all of them has 50K peers with most bogus.... and you will accept them from him as you always have.