Gen I isn't going anywhere for awhile. There area ton of great aftermarket parts for it, and it can still make power right up there with the LSx. LSX has a very small weight advantage over a iron block al head gen 1.
I also think it's a better 'looking' engine. LS takes a little more effort to clean things up, coil pack relocation, valve covers, intake etc.
Used Gen 3 stuff is super cheap though, guys are damn near giving that stuff away.
Not fair comparing the SBC to a flat head. The Gen I is much closer to the Gen III, then flat head to SBC.
BBC is still king.