Unfortunately, you "dumbing it down" is still stating the incorrect information. Your fault is that you're assuming everyone in the raid DPS' at the same level.
Let's say this is the DPS breakdown:
DPS #1 - 1,000 DPS
DPS #2 - 900 DPS
DPS #3 - 800 DPS
DPS #4 - 600 DPS
Tank #1 - 400 DPS
Tank #2 - 350 DPS
Healers - 0 DPS
Raid DPS is now 4,050 DPS. A 1.5% increase to DPS results in an increase of 60.75 DPS which then translates into 4,110.75 DPS.
Now, if we use your example that we increase the power of a single DPS by 12% and increase DPS #1 by 12% they're doing an additional 120 DPS and bringing up the raid DPS to 4,170.
Ergo, 1.5% DPS increase for the raid =/= 12% DPS increase for a single player.
I hate to be "that guy who nitpicks" but if you're going to argue balancing, it's very important you have a clear understanding of how specific mechanics and functions work and how they interact with each other so you can properly judge what is actually deficient and how to fix it.
Take Fey Wind for example. Selene provides a 1.5% raid wide buff. AST buffs SHOULD provide a higher potency buff to the raid for the trade off that you can't call on a whim. The fact that Expand Balance only provides a 1.25% raid wide buff and the chances of you actually drawing said combination is a 5.56%/min is terrible (if you have Shuffle available at the time of the Draw, you increase your probability to 10.19%/min). This is of course assuming you don't Spread a Balance earlier in the fight and work towards getting a RR'd card for Expand, which one its own right can be quite frustrating given RNG.
Likewise, take Balance. It's a 10% increase for 15 seconds. Say you get two Balances in a row or RR an Extend for a total 10% increase for 30 seconds (or a 5% increase every minute). Even with this luck and the above party example, the #1 DPS would only be increasing their DPS by 50, which still wouldn't match the power Fey Wind grants to the raid.
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Also, at the risk getting hated - I'm going to play devil's advocate. What Staris says is true if you make educated assumptions based on previous raid design and current raiding with Normal Alexander.
If you assume the following
-Heavy accuracy checks in savage in vein of how they were for BCoB
-Low HPS checks
-High HP checks (this is different from HPS checks)
-High DPS checks
Despite AST's lackluster toolkit, they bring more answers to the above assumptions than WHM would. If the combined HPS of an AST and SCH can overcome the necessary HPS check with neither of them running out of steam and WHM can contribute 0 DPS due to accuracy issues, then you're going to bring a AST who at least has two DoTs that are 100% accurate and party buffs that, despite their RNG nature, will at least do SOMETHING compared to hoping your Aero / Aero II / Aero III / Stone III actually connects.
I will also add this - the above also hinges that the accuracy check is as heavy as Staris' implies and the hinges on AST having the longevity to survive these encounters. If neither of these can be met, a WHM will be a better choice versus an AST.
Without knowing the exact meta S-E intends to do, it'll be hard to judge what will be the best "composition" though. Still, I can't fault Staris' logic when you think it through.
I forgot to take into account that oGCDs aren't affected by Haste too, so yeah, Fey Wind generally is less potent than 1.5% due to the amount of oGCDs specific classes would like to weave. Fey Wind and Arrow are absolutely pitiful for SMN compared to other DPS due to the amount of DoTs and oGCDs they have. Does Fey Wind affect the pet? If not, that's even less potent for SMN due to pet's not being affected.