I used to buy physical as much as I could, but nowadays it doesn't mean anything, so I don't care as much about it.
Flash memory cartridges die, even faster in cases of bad batches. Optical discs have disc rot (again, some worse than others). Many many games have updates, DLCs or patches that won't be on the physical medium. Plenty of games coming on discs have to be fully installed on the machine's drive anyway because disc drives are too slow.
Most indie games, including some of the best experiences out there, never get physical versions, or only very limited ones.
The only way to preserve is to duplicate and archive everything, even if it's not easy. Keeping original physical media as a souvenir is nice, but it doesn't achieve long term preservation.