Lol, that doesn’t happen at all. I mean you do go off on your own, but you don’t lose your gear. You’re still in the tutorial area.
I wouldn’t spend too much time upgrading green/common gear and do recommend trying to get the purple/rare over the blue/uncommon. Even then, with different weapons, it mattered more about which types of bows I preferred and worked best on which machines (I.e. don’t use fire against fire machines). Slight spoiler: there are times you might not have access to everything due to a quest or cutscene, but you get all your stuff super quick.
It’s an amazing game that I highly recommend! Definitely worth it!
Having played through the whole game and it’s sequel & DLCs, I’m not really sure what OP is talking about. You don’t lose progress or lose gear. Some early game gear might not be as good as you progress the story and you will need to upgrade, but that’s fairly normal with open world games like Horizon.