I don’t think your first point follows. It’s a bit like saying why do we need a safety net when we can stand on the tightrope. It confuses two different questions: how generous is a UK entitlement and how secure is that entitlement?
In the EU, those rights which derive from EU law are secure. Outside, they are one hostile executive away from abolition. (By outside, I mean fully outside. EEA states have to accept EU employment law).
Some think adverse changes are inconceivable. I think that represents a significant detachment from reality. For example, the present government and its coalition predecessor have diminished a number of employment rights. They also conducted the so-called “gold-plating review”, the express purpose of which was to look to see whether rights should be reduced to EU guaranteed minima. But even if you are somehow sure this administration would make no further cuts, I don’t see how one could be sure that they would not come thereafter.