I'm glad you posted this, because I always have the same thought when reading RM's posts about short mash times. Alpha amylase acts more quickly than beta. So in theory you could mash for a really short time, even at a moderate temperature, and end up with a wort full of dextrins. We keep the mash going because we want beta to snip those up into fermentable simple sugars. And if you mash low, it's most beta acting in the first place.
And beta clearly does not denature that quickly. If it did, one couldn't mash at 160F like Lagunitas does when brewing their IPA (their head brewer discusses it on a Can You Brew It episode).
So I think it's a little misleading to espouse these short mash times without issuing appropriate caveats about fermentability.
Unfortunately I bought a Schmidling MaltMill with a fixed gap. So I can't crush finer, and milling twice doesn't get me much extra goodness so I often don't bother. I definitely see efficiency improvements with 75 or 90 minute mashes, so I am sticking with that as standard practice regardless of how quickly the starches theoretically convert. I'm convinced that there is more to mashing than just strict conversion.