Beach is certainly amazing, but he excels at neither the 100m nor the 800m.
His 10.78 in Des Moines corrects to 10.88, which for convenience is 10.9--which, for a good decathlete his age, is just good. Wind helps, he had a +1.6 there.
A 10.9 while training decathlon means that if he specifically trained for 100m, he could relatively rapidly get around 10.7, and might maxx out at 10.6 or even 10.5-high if he is lucky.
That is not fast for an adult male competing in the 100m.
Also, isn't his 800m PR something like 1:47-mid? Hardly "elite".
That's why he's a decathlete, because he occupies a mid-range--credible at both the 100 and 800, relatively worse in the 1500, but not great at any distance.
Eaton is more interesting to me than Beach--for instance, I think that if Eaton trained exclusively 100m, he would be around 10.0x-high, which is smokingly fast. With the right PED's, he's a serious sub-10 sprinter, which is world-class. He's not sprinting because he's the damn decathlon WR-holder, and because he'd never beat Bolt & Company.
Note, however, that Eaton is as much worse in the 800m than Beach, as he is better in the 100m--Eaton's 800m is something like 1:52, which is a far cry from 1:47-mid.
So, you have
10.23 vs 10.88
1:52 vs 1:47-mid
This illustrates my point exactly. Both Beach and Eaton train primarily for form, being decathletes, and power (like sprinters)--they are not best characterized as middle-distance athletes--and the better they are at the 100m, the worse they are at the 800m, as a general rule.
There is no doubt in my mind--none at all--that if Beach lowered his 800m time to an international level, like say 1:44 or 1:43, his 100m would be well above 11.0 He's got some distance to fall, but my feeling is that he would end up being around 11.4-11.5 in the 100m, probably closer to 11.5, which is adding around 0.5-0.6
This makes sense, as he would no longer be training the power to accelerate over the first 50m--and, it is still a great 100m time for someone who doesn't specifically train 100m.
So, I ultimately disagree with your assessment that he will "improve both of those" to any significant degree, at anywhere remotely near the same time in history. And he needs more power for the decathlon, including upper-body and core strength--and that means body weight--and as soon as that goes up, he can kiss his 800m good-bye.