Hi Sam,
Thanks for your taking the time to submit your suggestion. At present WHMCS uses PHP's date logic to calculate by how much to move a Next Due Date forward. The monthly billing cycle option will increment forward by 1 month as defined by PHP. A discussion of this is located here: https://docs.whmcs.com/Billing_Logic#Billing_Cycles

A 30 day billing cycle could result in customers losing days over the course of a few years, as 365/30 =/= 12 months.

But if there is demand for this and this caveat is not a problem in that case, it's potentially something we can consider.