What you are asking asks us to evaluate the competence of the EU negotiators and the EU commission. As they will be the ones that we will be negotiating with.
If the EU is daft enough to put tariffs on our products entering Europe, they must know that similar tariffs would be put on their products: cars, agriculture etc. If they did so, everybody agrees that they will be doing it to try and punish the UK rather than economic reasons for the EU.
If that is the case - you seem to believe it is likely they will try to punish us for political reasons, why would you want to continue with stupid people negotiating your existing EU deals with the rest of the world which is expanding?
Surely it's a win win, if we leave and they aren't stupid - we get a good deal. If we leave and they are stupid, we get rid of stupid people negotiating our trade deals globally.
Issue is they buy 45% of our stuff, we buy 7% of theirs....not a good negotating start point.
They also can't be really nice to us as others may leave.