The old system certainly wasn't perfect. It scaled really, really weirdly.
I also found it deceptively hard to judge at a glance exactly how effective your weapons would work against an enemys' armour, especially when it was bound up with the miss/graze/hit/crit process. You say it's simple, but it's only simple on paper.
The old system wasn't good enough. Low damage weapons felt too weak. Armor felt too weak.
Better to tweak this one.