On this topic here is the deal:
Brazier isn't running sub 1.40
Consider that since Coe first ran under 1.42 back in 1981 it's now 39 years on. (insane to write that - 39 years and 1 month! or 14297 days!). In that time we stand with a WR 0.82 seconds faster and even more telling is there have been just 2 athletes (Kipketer and Rudisha ) responsible for ALL of that improvement. This is important because this single statistic shows how we quite conceivably are approaching the pinnacle for 800m running for human physiology as we know it (as in if there were multiple athletes at 1.41.0 + or - 0.2 seconds it's more likely we would see jumps and even legit challenges on the record much more regularly).
I'm not saying that this is a purely linear situation but if it was another 40 years from 2012 until we see another 0.82 second jump, then we are looking at 1.40.09 in the year 2052.
The evolution in human physiology you would have to be seeing from Rudisha to Brazier isn't a "few years" deal - it's potentially multi-generational, and even then it is quite conceivable a lot of us won't even be around when we get to the point a human runs 2 laps in under 100 seconds.
So the answer is he's not running under 1.40
Can Jakob run 1.44? Of course he can. Jesus if Craig Engels can run well under 1.45 then so can Jakob. He ran 1.46 in his first real tilt at it and if he stays healthy in the next 2-3 years it's a formality if he wanted to do it - irrespective of "targetted" 800m training . He's run 3.30.16 for 1500m for gods sake. Trust me - he can run 1.44.something