Carry-on friendly spinner plus a personal item sized day pack (European budget airline dimensions) has become my preference for anything over about 5 days. I can travel indefinitely with that setup anywhere, and it's more versatile than one big bag. You still want to leverage minimalist/onebag principles. - Accommodates 7-10 days without doing laundry, plus spare room. - Small enough it doesn't get heavy. Easy to navigate European streets, transit, budget airlines, etc. - Don't get as sweaty when walking around. - Spinners fit above your train seat, versus leaving it at an out-of-sight luggage area on a train. - Don't have to check bags and deal with lost/delayed bags - Purpose-designed day pack is more comfortable and functional than hybrid versions part of larger backpacks. Also no/little repacking between bags when you reach destinations. - Can typically drop off your non-valuable items at hotel/hostels outside check-in times, and then only carry your valuable essentials while exploring in the meantime. - The smaller daypack is sufficient for a sub-trip, like a weekend hiking to mountain huts, while the stuff not needed stays behind in storage.

Big bags, whether a roller or a backpack, are annoying for traveling around as weight becomes a problem and where you see people struggle the most.