Couldn't someone just generate millions of addresses and use up every possible variant?
Theoretically speaking, of course. What would happen if this happened and there literally was't anymore viable addresses left to be generated?
Although the namespace isn't infinite, it's beyond human comprehension. If you set a brand new, high end computer to the task of seeking a collision (what you are describing) you
might finish up before the Sun turns into a red giant and engulfs the Earth into heat death. It's unlikely, however, that said time frame is a concern.