Can I just Leave a Missing Molar and Not Replace it?

I've had a molar extracted (far back, lower left).  I'd like to leave it like that and not get an implant.

1.)  I've heard that the other teeth could shift --- how many years does that usually take?
2.)  I've also heard the tooth ABOVE it could actually fall out eventually ---  how long does THAT take to happen?  
3.)  Is there a "fake/artificial molar" device I could get?  If so, would it be like a "retainer" that fits into my entire mouth, or just on that side?  

thank you