If WW3 is nuclear, yes you are right! But that would be pressing the self destruct button for everyone and I personally think there are lots of other equally effective methods of warfare that could be life changing and that makes prepping very much worth while.
Remember the old castle sieges when the enemy surrounded the castle and starved them out??? Previous posters have already eluded to this but there are so many ways life could be made dire without bombs. Internet cables go under the sea, cut them off and no one has access to the internet, banking etc. How long could you manage without access to money? Its not like you can go into a bank branch as there aren't any.
Cutting off trade roots, we all saw what was happening during the pandemic, and countries were cooperating. We are no longer a nation that has veg gardens and cook from scratch so what will you be eating.....what about fuel supplies etc. Most medications are from abroad, cut them off and many are in deep doodoo.
Prepping is not stockpiling, its a balance of probabilities, just like when you know Christmas is coming and your normal monthly wage won't cover the extra expense, so you start putting something aside in advance....that's still prepping. Prepping isn't necessarily a doomsday scenario, I haven't got weapons and ammunition stored under my floorboards (although....😏).
I have a couple of weeks food and water stashed away, I have spare batteries, torches and a wind up radio. A simple wind up radio is good in power cuts. I lived in the countryside, we often got snowed in or had power cuts, so we had back up generators.
If an enemy managed to wipe out the power grid for a few weeks, how would you go on? I can't say I would be comfortable but I could get through the initial crisis and potentially support others too.