First of all, that “evolution” is stupid. I graduated class of 2019 and everyone still had backpacks. graduated college 2021 and guess what…backpacks. Not one used a freaking handbag? Books are heavy af and even though most are digital, you still have a laptop and/or notebooks. Also, It’s not a stupid question. I would just imagine it is a mix. Most people probably used the strap if they had a lot of books. Maybe some just carried it. Similar to now. Some people have actual backpacks, but some people use smaller sized backpacks.