Without the UK the EU will be weaker for sure, both economically and politically. So, you might ask, why are European politicians taking a tough line? Simple, they want to crush the right wing EU sceptics, a task which should be easy now that the UK Brexit referendum has been shown to be a disaster. For right wing politicians, demanding national referendums immediately following the UK result may have seemed like a good idea. The appetite for that may now be waning with the reality check which is now taking place. Tactically Europe should ensure that the UK is either forced out or stays in under the old terms. No need for them to re-negotiate anything. When the real pain of Brexit cuts in will be the time to organise referendums throughout Europe to re-assert the European ideals.
Of course it may not need to go that far. A new UK government or another UK referendum which actually faces up to reality would serve the same purpose, put an end to dissent for once and for all. Of course what the EU needs is either a reality check of its own to make it more relevant to its citizens or better PR. Unfortunately forcing the UK to toe the line will probably put EU reform on the back burner, at least for now.