Once I got 400+ upgrades and could summon a lazer beam of instant death that filled the entire screen while running faster than god, I tried running extremely far in a random direction, at some point it was very lonely because the mobs stopped spawning. This is the point I had to close the game due to the loneliness. My only suggestion is to add waves so that it isn't nearly impossible in the beginning to pick up the coins, or at least make them not despawn in such a short time. Bosses would be cool!