Comparing a runners ability to step down from 5000m to 1500m and 5000m to 800m is one of apples to oranges though. Like I said, 1500m is aerobically dominant, 5000m is aerobically dominant, 800m is anaerobically dominant.
A world class marathon/half marathon guy stepping down and dropping a world class 5k/10k is not surprising and usually to be expected. This is because they tax the same energy systems, despite being 10-20 miles shorter.
A world class 3k/5000m guy dropping down to run world class 400/800m times is unheard of, and Jakob trains like a 3k/5k runner, not a miler.
You also need to understand Farrah trained with a lot more speed than Jakob does. Mo did tons of work at mile/800m pace and even some top end work under NOP, so I don’t think it’s even fair to compare the two in that sense.
Walker and Scott didn’t improve their speed because they couldn’t period, they didn’t because if they improved their speed it would be detrimental to their 1500m endurance, why improve your 400m by 1-1.5 seconds if it costs your 1500m 3-5 because of reduced aerobic fitness and increased body mass?
I must reiterate my point, I guarantee you Jakob is capable of 1:43high to 1:44low if he cut his mileage down to 60, hit weights, plyometrics, and speed work. It just would be a fruitless endeavor, because he’d go from being a world #1 1500/5k guy to a world class but ultimately also ran 800m runner.