The steel in the bolt is of variable hardness. That's part of the problem. The earliest ones were the worst, and apparently the "newer" ones are less of a problem. So, the thing could run forever, or it could fail in short order. Swapping bolts on a M14-pattern rifle is NOT a plug-and-play type deal. They need to be fitted. Your options are as follows, in order of cheapest and least safe to most expensive and most safe:
1. Do nothing. Do you feel lucky? You might be. Or, you could set off a bomb by your face.
2. Keep the original bolt and just check headspace regularly. If you go this route, the key is to check it REGULARLY. Like, buy a set of GO, NO-GO and FIELD gauges and check it each range trip. If the headspace doesn't change, you're fine. If it starts to get excessive, DON'T SHOOT IT. It won't get better and once it starts to go it will go pretty quickly. Move on to #3. The chance of it going from 100% in spec to catastrophic failure in one range session is probably vanishingly small, but the chance of it going from kindasorta/almostbutnotquite in spec to kaboom in one range session is high enough that I wouldn't want to fiddle with it.
3. Have a USGI bolt conversion done. I recommend Warbirds. He will fit a USGI bolt to your receiver and you're back in business. I can't remember how much he charges, but the charge for the conversion will cost less than the USGI bolt.
As was said, the receiver is fine. It is actually a VERY good receiver, made to proper specs of exceedingly good steel (better than most anything being made today, including SAI). The problems with the M14S are the bolt and some of the small parts.
Full disclosure, I have SAI and Polytech rifles. I'm hard pressed to decide which is "better". I think that the Polytech will probably last longer, but at the current price of ammo, I doubt I could afford to wear out either one.
Mike