T2 and higher Alphas improve multiple attributes. You're correct, they do apply those boosts only to applicable powers, but the fact that they're improving three or more attributes and doing so globally means we gain the equivalent of many extra slots. If I slot Agility Core Paragon on my Illusion/TA controller, I get a free Defense SO in Combat Jumping, Hover, Evasive Maneuvers and Superior Invisibility, and a free EndMod SO in Stamina, and free Recharge Reduction SOs in Blind, PA, OSA, EMP Arrow, Spectra Wounds, Poisonous Ray, Hasten...
You're trying to make the case that Alphas can't be considered the equivalent of extra slots because they don't necessarily always boost what we want boosted, that it "doesn't count" if you can't slot ten different things in ten different powers, but that's not relevant because Alphas aren't binary and we don't treat them as such. That SO's worth of +Whatever, +Stuff and +AnotherThing is the equivalent of one or more extra slots in every relevant power, and since we can choose among a wide variety of +Whatever/+Stuff/+AnotherThing options to gain the most benefit to the widest range of powers, we can tailor those slot equivalents to a significant degree.
You'd have a point if Alphas never progressed beyond T1... but they do. Or if players slotted Alphas which only buffed one attribute for a single power in an entire build... but they can't.