I posted a longer response, but the continuing d****d trouble with the server ate it.

I agree with the interpretaion that the Servian hoplite system probably ended sometime in the 2nd Samnite war. Terrain and tactics of the enemy must have been responsible. The Samnite maniple and duplex acies system, combined with their light infantry, javelins, cavalry, and heavy infantry would have been an ideal force to fight on such ground. Light forces do well vs. hoplites in covered, broken, and mountainous terrain, while the heavies would give the ability to hold points where needed. A similar though less fundamental change had occurred in Greece in the Peloponnesian wars. Athens suffered in some forays into such terrain, but learned from the mistake and employed the same light forces method to defeat some Spartan forces. In this case light forces were shown to be excellent at destroying hoplites in difficult terrain.

The more I've considered the Servian system, and its force distribution, the less I am concerned with the idea of the bulk of it being in the 1st class (wealthiest other than equites). All hoplites would have been land holders with a vested interested in protecting the countryside. It makes sense that they would form most of the forces and be similarly heavily armed. This seems to have been the hoplite ideal as I understand it (perhaps improperly.) It also carries on with the earlier ideals of warrior elite being the nobility. And hoplite influence is suggested in the later forbidding of consuls to lead the army mounted. (I think it was sometime around the 1st Punic War that this was dispensed with.) Note the Servian system did have a small contingent of slingers and javelinmen, nothing really novel about that. There is little reason I think to be surprised that the lesser levels of society were greatly under represented at the time.