What happens if you have followed plan A but then the weather turns nasty? Lets assume for the point of the exercise that you are IFR.
Note I said pick suitable wx.
By suitable wx I mean wx that suits the aircrafts capabilities. That is if it's a VFR aircraft your wx requirements are going to be much better/higher than for IFR.
Likewise if you're IFR but don't have the endurance to do an approach then divert, your wx requirements will be higher than if you had the luxury of being able to do an approach then divert.
For anyone with out the ability to divert after an approach PNR really is a critical time any doubt at this time a return should be done.
Forecasts, Metars, seasonal wx, the advice of the local met guys etc should all be taken into account when deciding when to go back or when to continue.
If you get all of this info, änd plan according to your capabilities, you are extremely unlikely to have a "What happens if you followed Plan A but then the wx turns nasty" moment.