Saw a DIY shed base in Portland that made me question everything I do

I was in Portland last month helping my brother move and his neighbor had just finished a shed base poured on nothing but 4 inches of compacted gravel, no concrete, no pavers, and that thing looked solid as a rock when I walked on it. Meanwhile I dropped $400 on a concrete slab for my 10x12 shed back home and spent two weekends digging out roots and leveling the form. The gravel guy said he's had his shed sitting on it for 9 years with zero movement, not one crack in the walls, and he just raked it level with a 2x4 and a string line. My concrete base is heavy, permanent, and if I ever want to move the shed or sell the house, it's a giant slab of regret in the yard. But I keep thinking about how one bad rain season up here in the northwest could wash that gravel out from under a corner or shift it when the ground freezes. So what's the real answer here, are we all just pouring concrete because our dads did, or is gravel actually the smarter play for a small shed? Anyone actually run gravel long term and had it go bad?