They already said that the pages taken by the reprints are not replacing any possible new crunch. If they didn't reprint the SCAG stuff they would not be adding more new things, only lowering the page count.
Don't WotC books have to conform to a a number of pages multiple of 16? I don't think 4 subclasses will take more than 5-6 pages.
There is no way around it really, if they add something to a book, it takes up space that could be used for something else, even if that would be just larger artwork. Now if they don't think they have enough material to fill the page count, they will fill it with whatever they have at hand, zoom some pictures, or leave the pages blank. Which one would games appreciate more?
They must have thought about pushing more stuff from UA (or something we've never seen), but presumably they believed that everything else was too poor, half-baked or risky (such as prone to abuse) to publish. They could have put the Mass Combat rules in those remaining pages for example, but if they don't do it, it must be because they don't want to "burn" them by releasing an unsatisfying version, they still want to keep working on them until they can publish something that won't need further updates.
Didn't this "republish" controversy happened also in the AD&D era? It is totally understandable that someone who bought a book and sees part of it pasted to another book they also plan to buy, would feel cheated into spending 100$ for maybe 90$ worth of material, having to buy the same 10$ worth of stuff twice. OTOH filling up those 10$ book space with 0$ worth of stuff isn't a good idea either. The only good solution would be to just do some extra work and increase the publishable UA material a little bit.
About the AL "PHB+1" rule... this is really only related to
player characters stuff right? DM rules modules don't really matter for this purpose. So when they say they want to help players in AL by reprinting
four SCAG subclasses, we're really talking only about a player who wants to use
feats or
spells or
magic items in XGE together with one of those 4 subclasses (or maybe a multiclass mixing up with a XGE subclass). Basically these are specific
combos. But isn't the AL rule
meant exactly to limit the number of combos? The results of limitations are always arbitrary... there's no way to tell that a Swashbuckler + XGE feat should have more or less rights to be in AL compared to a Purple Dragon Knight + XGE feat.
So despite of the fact that personally
I (who didn't buy SCAG) can only benefit from those reprints, I'd just say
suck it up and live with that limitation! If you want to play
any combo with
any books, do so in your home group. I can't believe gamers would be so petty to pretend exactly that combo in AL, you're lucky enough that there are AL games to play in the first place.