As kijangnim has pointed out, the 'experts' are tending to confuse INS/IRS update with position update. Two separate issues. Probably confusing A320Slave too!
Most 'old' systems were indeed capable of position update. The alignment for a platform requires steady conditions over several minutes which can rarely be achieved in flight. Normally the best 're-alignment' in flight gives a fairly crude attitude reference. Often it requires a repeat at intervals.
I do not see any reference to 'OLD' systems in the post (just 'no GPS'), so for A320Slave - it is by no means unusual for an 'oldish' (no GPS) IRS/INS to be aligned at start of day and left untouched all day for several sectors. No loss of navigational (or indeed attitude accuracy) will normally occur in a 'rich' environment where the position update is easily obtained and kept, and most systems can update position at take-off on a defined runway as well. Even with the 'errors' IGH mentions, the resulting accuracy of navigation is still streets ahead of the old star/sunshot/Doppler drift/LORAN/piece of seaweed systems. The only time when it is really important to align accurately is before a longish flight in a region where updates are not available - eg ocean crossings. Of course, now-a-days the REQUIREMENTS for navigation accuracy have tightened, so frequent re-alignments are more advisable.
The really 'old' INS on the early Harriers was position updated by 'pickling' over a map point, but once the platform had lost its alignment and had rushed off into 'enemy territory' at 200kts, there was little point, so we just turned the map off and got lost manually.
