But when the process improves and the end product is the same, it just takes less money/time to make it. So the only way common people would benefit from it is if manufacturers decided out of goodwill
The industrial revolution improved the process. Before that, for example, knives were traditionally made by a skilled blacksmith and were very expensive. After, they were made cheaply and much better and made their way in every home. Just like pots and pans. And clothing and carpets and chairs and literally all the goods which required a skilled crafstmith and were expensive and scarce became massed produced and became cheap.
Same with computers: things that were hard to make because they required skilled workforce became easy to do and cheap with automation.
It will be the same with AI: another round of things that are expensive because they require skilled labour will become cheap and available to everyone. This time it will be even more complex things than before, things that require a bit of ingenuity like medical diagnosis, maybe driving, maybe teaching, maybe writing (but more probably editing rather than writing). Think cheap basic healthcare for everyone. Think free, good, reliable public transport for everyone. Think reliable press. I don’t know what form it will take and where we’ll find applications for it.
It’s not clear what capabilities this technology has at the moment and what is its future. However, it promises a wonderful thing: the ability to scale up for free things that couldn’t be scaled because they could only be done by people and people are in short supply.
As for the work hours comparison between now and the medieval times, that comparison is not correct. It compares working hours, but doesn’t add in the effort required for just living. When work is done, you have to make food from scratch always because you can’t store it for too long, gather firewood, clean the firepit, bring water from somewhere, make tools, make clothing, wash and clean the house, constantly repair a host of poorly built things that require attention, a million things to do always. We really can use our down time for leisure nowadays.