Good post to revisit.
The answer is still the marathon - even if you want to discount Kipchoges exhibition.
1) A human being has never run 800 meters in under 100 seconds but a human being has run under 120 minutes for 42195 meters. Yes there may have been "aid" to that (as there is on the track with the track surface, wavelight, pacers as well) but a human being has actually covered the distance in the "target" time with their legs, lungs, heart etc etc.
2) To get to 1.39.99 the current 800m WR has to improve 0.9% To get to 1.59.59 the current marathon record has to improve 0.5%. That's quite a big difference.
3) Yes we have a great crop of 800m runners that redefined the sport this season. But still, despite a number of really quick races, the reality is still nobody broke 1.41.0. And ironically, I think the top heavy depth of talent in the 1.41's isn't a massive plus factor for that. Yes on one hand the hot competition helps, but it also creates competitiveness in terms of actual race execution and position on the track and this doesn't always spit out the best times.
One of the reasons Rudisha was able to run as quick as he did is because basically every one of his competitors in that era just left him alone to do his thing. He never had to worry about getting out too fast or surging mid-race to hold his position because nobody was ever there. London wasn't actually a great race - it was a great series of individual time trials. When they hit the break line in that final, nobody was messing with David and this helped him run the race with the splits and execution that he wanted. 2-3 guys with the ability to all break 1.41.0 all want the same thing - to get out more conservatively than the "fast" 800's of yesteryear (the days of sub 24.0 second opening 200's of the Kipketer era are long gone) and they want to run the shortest distance in lane 1. But only one guy can - ask Arop who ran 1.41.20 with 600m of it in lane 2 trying to get past Wanyoni in Paris.
4) With the tragic death of Kiptum the guy we all thought was a foregone conclusion to break 2 in a regular thon is gone - but in the same way he kind of came from nowhere (no high profile track career like Kipchoge, Bekele, Geb etc), there is no reason why someone else couldn't do the same. On top of that we have a golden era of top track talent most likely coming to the end of their careers - some of them who would seem perfectly suited to the marathon (ahem, Cheptegei). The chances that one of this crop (Kejelcha, Barega, JC etc) make a successful transition to the marathon in the next 5 years is quite high.
5) Final thought on bicarb. Pretty clear the gel is doing something - it can't all be placebo effect. But in saying that, guys are using it now to good effect. Maybe along with smarter race execution it's helped Arop (a big, muscular guy) run a second faster than he might have had he been running in the early 2000's - but he has access to it now. Is there a Sodium Bicarb gel 2.0 that has twice the effect out there? I don't think this is making 1.45 runners 1.41 runners. If we think this is going a be a big reason why 100 seconds in broken then we need to be looking at 1.41.0 "non-bicarb" guy in the very least. Hoppel claims to have not used it and he's at 1.41.67 - does a Maurten gelled-up Bryce Hoppel improve 1.7%?
In my opinion we are still are considerable distance from 1.39.xx - and far further away than we are from a "legal" 1.59.xx marathon.