I think the switch to the 3-4 was inevitable. This is a copy cat, cyclical league. Something's only a fad in the NFL until it sticks, and the shift to the 3-4 appears to be in full swing.
it's funny b/c the 3-4 gained momentum like 10 years ago or whatever b/c it was economically useful to implement
rush LBs, 3-4 DEs, and slower/bigger MLBs were not valued highly inthe 4-3 so teams could get them for cheap if they ran the 3-4. even after they were stars, there wasn't a market for them in the 4-3
it's going to swing back at some point, but the rules changes seem to make the 3-4 more preferable right now