The two reasons why chainline is not required to be perfect on a geared bike is the bushingless chain that permits the side flex, and the use of a derailleur which aligns the chain accurately (assuming correct adjustment) in a vertical position when it is engaged by the cog tooth.

Bushingless chains are used on fixed gears, too, so their inherent side flex does, to a degree, overcome the issue of perfection that you say must exist. I said in my post "poorly aligned fixed gear"; in the absence of a derailleur on a fixed gear, the limits of misalignment admittedly are somewhat narrower than on a geared bike... I've always only ever eyeballed the chainline on my fixed gears, and despite this imperfection, have never had a thrown chain.

The point I was trying to make is that inappropriate chainline causes problems on both fixed/single-speed and geared bikes. A poorly aligned fixed gear illustrates the effect of poor chainline by throwing the chain when the alignment is outside certain limits.

A geared bike with poor chainline means cross chaining can become an issue at one end or the other of the cogset, even with bushingless chains, because it, too, will throw the chain, particularly on the chainrings, when the chain passes its reasonable limit of flex.

A chain that is worn on the shoulders of the side plates is likely to be a little more capable of handling the limits of chainline misalignment, but other issues would come to the fore because of other wear factors.