Even a 20% fluctuation would make it unviable for a person who needs to make long-term planning like, you know, paying rent and buying food.

Last week alone bitcoin price fluctuation was ~8%. Last month? 146%

146%?

Any source on that? It would need to go from 20K to 50K.

My bad. It looks like I misclicked on some graph here: https://www.coindesk.com/price/bitcoin/

But we could take past year as a whole. Fluctuations from 47k to 16.4k (3 times drop) then 27k (1.6 rise, still 1.7 below the high). There are drops that go to a price 1.5 lower in just a week. Imagine you need to pay rent on that week.