I have never had a WN delay of 30 min. end up going out at original ETD but obviously I'm the exception. Unfortunately it is impossible to predict unless you know where the in-bound aircraft is currently located (which sometimes is easy to determine, if you check the arrivals board...) While an airplane swap is possible, I find that highly unlikely for short delays given the scheduling hassles - more likely for longer mechanical delays.

Just 2 days ago I showed up to board a UA LAX->SFO flight and was told there would be a 5-hour ATC delay due to rain at SFO. About 15m later we received a text that the flight was boarding - ended up in Group 3 instead of Group 1 (of 5) but nobody was complaining. So you just never know about ATC delays.