I don't believe anyone knows for sure how they would react, but our reputation as a country that upholds it's agreements would suffer. Giving other parties fair warning that one has decided to secede from the union, and putting into operation a short process to do so gracefully, is one thing; just stopping dead is quite another, and it would take a while all other nations to trust that we weren't going to just act like employers and tear up a contract when it suited us no matter what the cost to others; again.
Even though trade is balanced with us importing more from the EU than we export to them, it doesn't mean that an EU that feels disrespected would automatically do the right thing and continue to trade unchanged. We are all human with human failings, and it may decide to retaliate anyway. It may decide to take the hit and tear up trade agreements, and since all countries need trade to prosper it's not a deteriorating situation anyone should want to risk.
The UK is an honourable union, it will negotiate an exit from the unfair and deceptive agreement that was forced upon us by the likes of Heath and others who, when the referendum to stay in was held, claimed it was merely an economic trading agreement. And other nations' governments can keep their opinions to themselves as they would only be looking after their own interests, and not those of the UK.