I'm able to organize well enough to allow the part of the company I'm responsible for to work 30 hour weeks and go home.
If I do this, executive management will conclude that I'm overstaffed. They'll cut my personnel budget, sending the savings to profit. Many will become unemployed. The survivors will go back to the 50 hour weeks they worked before we achieved greater efficiency. The shareholders will be happy with the bottom line increase.
This is how it is under capitalism when working people are unorganized. The owners of capital appropriate 100% of the productivity gained through technology or intelligent management.
In countries with powerful union movements the outcome can be different. It's possible there for productivity gains to be shared between capital and labor. It all depends. There'll be a tussle, and the stronger each side is, the more they'll receive. This is why Germany has a 35 hour work week today.
This is the crux of what Marx meant by "class struggle".
With socialism we could make the distribution democratically. As a people we could decide to make it our societal goal that everyone should work, say, 24 hour weeks, paid as before. This could be collectively debated and democratically decided. No-one would become unemployed, because the benefits of improved productivity would be shared. Living standards would be maintained, but quality of life significantly improved for millions of people.
Why don't we do this?
Because this is the crux of what Marx meant by "class struggle".