This is why I eventually stopped trying to be a completist, with a few exceptions. It didn't make sense to spend hundreds of dollars for a key date that otherwise looks just like the rest of them, just so I can say I have a complete set.
I understand, but I tend to think the "darkening" gives it a bit of historical gravitas that otherwise might be missed, if only in comparison to this wonderful specimen.
I face that conflict with just about every series I try to put together! For this particular series, 95% of the stuff is available for just a few bucks. If you skip the proof-only issues, very few of the coins should run more than $100. But that single 1906-S is well over a thousand. Is filling that lone remaining hole in the set worth it? It's kinda like me trying to put together a SLQ set. Every date is extremely affordable. But that 1916 ---- wow! So do I even start knowing I'll probably never complete it. It's the great conundrum.
I understand your predicament, and of course it remains a matter of personal choice as to what and how much you are willing to sacrifice otherwise to fill that solitary open slot. I'm not sure if this will mean anything to you since it's definitely more philosophical than numismatic, but I often take what I consider to be more than sufficient solace in the memorable words of the immortal Leonard Cohen, to wit: "There is a crack in everything; that's where the light gets in." Which is to say, perhaps the missing coin is the but the space that lights up all the others.