The Global Inbreeding Epidemic

Could a simple thought, foresee an ironic end to our race?

I've been thinking about this for a little while. You have three couples of a man and a woman in a room with no doors. Lets say that theoretically they can live without food or water. The only thing I'm getting to is the urge to procreate occurs, and they begin to have children. No one is allowed to leave. Ever. The children get older, and they of course have the urge to procreate. The young men and woman have children. This continues for however long. The parents have children, and their children have children, that have children..... Imagine whatever you want. At some point, that room is going to get pretty full. And what will happen at some point? There is going to be some type of unintentional (hopefully not intentional) inbreeding inevitably at some juncture.

Now think of the Earth as the room with no doors, and instead of a few couples, there are thousands, or even a hundreds of thousands of couples across the globe. This would be the start of the human civilization. Now we began to procreate for a couple thousand years. Things are getting pretty tight in the room lately, right? Is there anyone who keeps a tally on the occurrence of unintentional inbreeding? Does that even exist? Yeah, I know, I'm ridiculous.

Sooner or later, the gene pool is going to get pretty tight, and begin overlapping. There isn't any fresh gene pools coming in from anywhere else, and we all come from people who were since the beginning. Now give us a few more thousand years from now. Who's to say that when the population rises predictably higher, and if we escape any number of unforeseen tragic occurrences to our planet, that we would actually begin to inbreed, ironically by our own instinct to survive. Are we just naturally devolving, and just keeping busy while it comes? I know. You hate me.