No being louder just increases the number of neighbors who's networks you slow not how slow they get. Your old rounter will stop talking for a random interval if it hears anyone else talking so it wouldn't completely jam anyone. Furthermore your immediate neighbors would probably switch to a different channel. If you got lots of routers on all the different channels and a ton of random devices connected to them sending data you could really slow things down. WiFi slows with congestion. Being louder increases the range you congest.

If it worked the way you described WiFi in apartment buildings strait up wouldn't work. Which admittedly WiFi in appartment buildings doesn't work well.