How much does Toei even pay their employees? I hear it isn't all that much anyways, so all the extra stress on the staff can't be good for their health or creative levels.
It depends on the individual. Generally speaking for the entire industry, in-between animators are paid about $2 a drawing/cel and can handle about fifteen drawings a day ($30 USD a day). Someone who has just begun working as a key animator makes about $40 per cut (shot) and can usually do two-to-four cuts a day, depending on the complexity (the average episode has three hundred cuts). This is how money is spent on a regular television episode of a series. Prices for an average episode tend to run at $100,000-300,000, although those higher figures are not always likely. I think One Piece stays around 100,000 an episode, but that's just a gut feeling considering how few drawings and talent are employed by the series. Tengen Toppa Gurren Lagann had a budget of $180,000-200,000 an episode. Thanks to the talented key animators and directors employed you can really see all of the money up on the screen in every episode.
The nice thing about the industry is that hard work and talent is always repaid in kind. Iwane Masa'aki of Studio Cockpit has been with Pocket Monster since Episode #3. By Episode #59 he earned the right to provide all of the key animation for episodes himself. He's traded on-and-off over the years, but for the last three or so years he had kept to providing all of the key animation for an episode of Pocket Monster Best Wishes and Pocket Monster XY himself for every four-to-six episodes. That's at least $10,000 an episode, to say nothing of any money he makes from also taking the animation supervisor credit. By all signs Iwane has more than enough money to retire at his age, yet even despite his heavy schedule he continues to pop out solo episodes and even contributes to other episodes in the rotation. Iwane is the series' top regular action animator, too. Yokoyama Kenji used to do solo episodes on One Piece, but the series' production schedule has become so much shorter that in recent years he has other animators to work alongside him.
Of course it is, but One Piece's complete story is all of the episodes, One Piece won't be complete with just a single disc. If Toei didn't release with a DVD format, you wouldn't need hundreds of discs, either.
These specials tell a single story with a beginning, middle, and end. They're not a big commitment. A six hundred episode series some lowest common denominator viewer will likely never watch again is not going to sell to many people.
Naruto, Attack on Titan, and JoJo's Bizarre Adventure are not cheap, disposable trash. Maybe because they don't spend as much time making cash grabs and millions of movies?
Shingeki no Kyojin: Attack on Titan was made in a mad-rush and aired six months before it should have. Episodes were airing in complete, missing broadcast times, and generally would tossed together in four-to-six weeks with over forty animators, a dozen animation supervisors, two or three episode directors and multitudes of animation studios credited as having worked on each episode. That tells you something about the production committee's appreciation for their work.
Naruto isn't made by a company working on a seven weekly series and films at once.
JoJo is a late night series aimed at adults willing to pay big money for a property that isn't entirely mainstream friendly.
Making films and specials are a perfectly valid and intelligent business move for long-running franchises. Need I really mention your average schmuck doesn't want to own long TV series because of how little space is available in Japan?
Toei knows that One Piece fans will always watch the anime, however bad it is, because it is One Piece. They only care about the paycheck and not about the quality of their product. They will use whatever trick they can to get more money. You are talking as if increasing the quality of the series is not an option because it is not going to increase their ratings, and that is true, but Toei have the money and they will not lose a dime by doing so. They can create a series with much "higher prestige" than it has now, and spending that money on specials and movies is not going to make that happen.
There is no intelligent reason for the production committee members to place more money into a TV series that is already six hundred episodes long. It would be financially irresponsible, especially when you can make shorter and better things with your time and money.
Fuji Televi orders a special. That is the mistake right there. They know they have the money to make good anime, so they waste it on cash grabs, instead of the actual series. They recreate already created material to make up for their "mistakes" in the past, even though the "mistakes" were a product of their low budget! The system itself is hilarious and their strategy seems to be making the series as awful as possible without losing any viewers whilst saving as much money as possible so that more people will buy the recaps, thus giving them even more money in the end.
Making a high-quality special or film isn't a cash-grab. Dragon Ball Kai is a cash-grab. Something like Episode of Nami is a well-produced work of art one can enjoy with relatively little commitment, especially considering the national fad status of the franchise.
One Piece is a franchise for the lowest common denominator. A cartoon series was created in the first place to sell more volumes of the comic and other merchandise. It exists today solely to push products that are more desirable. A six hundred episode cartoon is not desirable.