However, a decrease in productivity means that stuff like clothes, transport, food, IT services, and pretty much everything you can think of that someone has to produce becomes more scarce.

Would not having 30 dresses make you unhappier, if you have time to spend doing things you enjoy instead of consumption being the only thing you have to show for all the time you spend at work?

How much transportation is actually what we need for living and how much is induced by being forced to go to work?

Food has the amazing ability to just grow with limited human intervention, so there is no reason to assume a reduction in food availability. Also with more free time people could tend to a small garden for some of their food more easily.

IT services... You are on a platform run by volunteers in their free time. More free time would mean more of such services available.

Capitalism has outpaced "intrinsic" consumption since at least a hundred years in the industrialized nations. Most consumption is induced by advertisment and social pressure manipulating us to consume more, so we work more, so we consume more, so the rich can extract more wealth in every cycle for themselves. You cannot separate wealth distribution, scarcity and work time from each other.

For the average people i'd wager the available goods and services wouldn't change much, as the people who make goods and services exclusive to the super rich like yachts would be producing other goods instead.