If everyone makes their own currency like you said, then each one of those currencies will be worthless. Who's going to want to take your currency when they have their own?
100% correct. Right now we have more than 10,000 altcoins and tokens in existence (this number includes forked coins). Most of them are not even listed in coinmarket.com and are not traded actively. If everyone produce their own coins, then the situation will not be much different. The vast majority will die out in a week or two.
Most of the altcoins are just trash. These developers keep on creating altcoins and advertising it like it does make sense for the people to buy but in the reality it is just a useless coin for the people to buy and once they get enough money from selling these altcoins, they will just leave it and add it to the collection of trash altcoins.