Well let's have a look at what it would take.
These are the final 200m splits from the 5 fastest 800m runs in history
Rudisha London 26.61
Rudisha Rieti 26.42
Rudisha Berlin 26.55
Kipketer Koln 26.5
Kipketer Zurich 26.9 (he did go through in 48.3 so understandable and will help us later)
So I think that we have a pretty good idea of how fast this final 200m can plausibly run. Let's just say for the sake of this that the final 200m would be run in 26.3 seconds. Any faster would seem unusual - especially given that the intermediate times at 4 and 600 would have to be considerably faster that what we have seen already and any slower would mean there were some ungodly splits through 4 and 6 that would even supersede what we saw in Zurich 97 from Kipketer.
That gives us 1.13.6 at 600m. To put this in context, there are only 5 men in history that have ever run this in a race, let alone an intermediate split where they still would have 25% of the race to go. It's only 8/10ths off the WR of Gray. Now I have no doubts that London 2012 Rudisha would have been under this mark (1.12.81) - probably somewhere in the 1.12.2/3 range and in London he hit's 600m exactly 2 seconds slower (1.14.3) and that is a pretty standard conversion between what a top 800m can run for a 600m all-out and what they can manage for 600m en-route in an 800 without utterly imploding in the final 150m which means our sub 100 guy also needs to be a 1.11 mid 600m runner.
1.13.6 at 600m with a 3rd 200m approximately a second faster than the capability of the final 200m (also a very normal pattern we have historically seen) means around 48.2/3 at the bell. Now let's think back to Kipketer in 97 who split exactly that. His next two 200m split's coming off this insane pace were 26.0 and 26.9. Now I'm not saying Kipketer was the final boss of humankind 800m running, but he was pretty damn good and I still personally believe that his potential, especially on this night in Zurich, was a lot better than what he ran (like if he went through in 49.0 he could have run 1.40.7X) - I just find it hard to envision someone this season going through at that pace and then running a low 25 second split followed up with a low 26 second split.
Not only so I find that hard to envision this season, I kind of find it hard to envision in my lifetime to be honest. I feel like the candidate to do this has to be a 44.5 400m talent (relay is fine - similar to David) and a low 3.30 1500m runner. Or imagine Emmanuel Korir who ran 44.2 at altitude also having the endurance to run a 3.33 1500m. I just don't know if/how you develop that threshold and endurance without compromising your anaerobic power that you need to run that fast.
In 1981 Coe ran 1.41.73. The record today nearly 43 years on, sits at 1.40.91 which is 0.82 faster. Given how few men we have seen even challenge this time from over 40 years ago (there are still only 4 others to ever break 1.42 and "one" (^^) is a suspended doper) I think we can safely assume we are pretty close to a limit for human physiology as we know them - like we are surely close. Even if we look at 2012 when the record was set and benchmark it v Coe in 81, that was 31 years to drop 0.82 - I don't see how we aren't at least in the same ballpark (and honestly even more) to go another 0.92 seconds to get to under 100 seconds.
Simply put, I doubt even if there is a candidate to ever run under 1.40.0 for 800m that they are even born yet.