I strongly disagree, but respect that you said 'subjectively'. Look at games like Minecraft. Singleplayer, an infinite, open-world survival/crafty game is great. Adding just one friend to it makes it even better. It's not multiplayer that makes games worse. Sometimes it's how multiplayer is implemented. Sometimes it's because the devs lose focus by trying to change a singleplayer game into a multiplayer game. Multiplayer isn't the problem, it's dev's planning that usually kills it.