Price will come down, quality will come up. The first VHS/Beta machines were north of $2000.00, a color printer, 1000.00. So what if a printed AR15 lower will only last a couple hundred rounds. You print off another one and swap it out.
3-D printers have been around for at least 20 years. Any of the "plastic" printers would have a catastrofic failure 1st shot, if you could get the lower to hold together for assembly.