I'm not a leaver, but we don't owe our working rights to the EU, we owe them largely to the efforts of people such as yourself, in the unions. This particular right is a case in point:
The rule is already honoured in the breach as much as in the observance. I'm not saying it's not important, but it's as a negotiating tool rather than some kind of absolute protection. Unions should definitely fight to keep it, but if they can't it will be because they are too weak, rather than because we've slipped out of the EU's benevolent care. There are no workers' rights that aren't won and ultimately protected by workers.