5E isn't modularly designed.3.x lacked the modular system design at the core of 5E. It is very easy to unplug rule elements and replace them without disruption.
It was initially proposed that it would be, but that wasn't actually followed through. So this isn't the case. It has modular elements (like Feats), but we're not talking about a system like FUZION where it's fundamentally modular.