There's a major component that you're overlooking though: They want to destroy the value of all the books they've already sold their customers. It's the difference between you releasing new miniatures because you want to sell me more miniatures, and you coming over to my house and melting all of my existing Reaper miniatures into slag so that I have to rebuy them all. It's a kind of sleazy business tactic, which is why it annoys so many people.
Actually, an even better analogy than you coming over to my house and slagging my miniatures is this: Reaper decides to stop producing miniatures in the 28mm scale and start making them in a 35mm scale. The idea being that if I want to buy the new Reaper miniatures, I have to scrap my existing collection, because my 28mm models will look scrawny and small next to the new 35mm models. So rather than release new dwarf models that offer me a chance to expand my existing collection of dwarves, you instead try to re-sell me Norg Kegbreaker in a different scale.
Of course Reaper can't do that - it would be market suicide. Your customers would rebel and refuse to buy your miniatures. There are plenty of other miniatures companies out there, and many of your customers buy from a wide range of vendors, so if you decided to change your scale dramatically you'd just see most of your customer base abandon ship. The pure painters, who don't game with their minis, might not be upset, but it'd still be taking a chainsaw to your customer base (and thus your profits).
But WOTC can do that (or at least thinks they can, Paizo's Pathfinder may have proven them wrong on that point). They can do that because the current edition of D&D has always been the most popular RPG and thus the easiest to find players for. Many people (especially GM types) prefer a system other than D&D but still play the current edition (whatever it may be) because its more fun to play D&D 4E with six people than Burning Wheel with none. It's kind of like how I'd rather not have to spend the ridiculous amounts of money Games Workshop demands from Warhammer players, but I do anyways because it's a 1000 times easier to find people to play Warhammer with than any other miniatures wargame (I'm lucky that the manager of my local GW store is kind of clueless about the history of GW and so I manage to use a lot of (frankly massively superior) Reaper minis in my Warhammer armies by claiming that they are old out-of-print Citadel figures. Shh, don't tell anyone).
Anyways, that is where the outrage comes from. Now, in fairness, it does seem that the splatbook model of expanding the game isn't commerically viable (or at least isn't profitable enough for WOTC's masters at HASBRO -- clearly it is viable on some level, as its the model Paizo uses and it has worked very well for them, mostly because they offer subscriptions to their supplements), but that doesn't change the fact that many people are content with the existing edition and resent having their arm twisted into re-purchasing the Player's Handbook, Dungeon Master's Guide and Monster Manual for the third or forth time. It becomes particularly aggravating when you understand that this is the new business model. Why spend hundreds of dollars investing in 5E when 6E will be coming out in 2016? It feels very much like signing up to get ripped off. And it also feels like WOTC is the 800lb gorilla getting its way by throwing its weight around.
But I'm not going to get upset about 5E, because I already abandoned ship with 4E, and frankly its easier to find Pathfinder players in Seattle than 4E Players, much like I expect it will be easier to find Pathfinder players than 5E players. WOTC so completely screwed the pooch with the release of 4E that I actually suspect that 5E is less an attempt to force everyone to buy new books (that was the motive of 4E) and more of an attempt to stop the massive hemmoraging of their customer base over to Paizo. But I frankly think its just too late for that. I think too many people have too much animus towards WOTC at this point to ever trust them again.