Was Dragonlance the first real adventure path? There were certainly sequential adventures before that, but was that the first full campaign series (level 1 - max) linked by a single plot?
Intentionally? I think you are right, but as has been pointed out many times Temple of Elemental Evil, or The Slave lords modules, combined with the Against the Giants, Vault of the Drow, and Queen of the Demonweb pits series certainly works well as a AP.
As far as a business plan, I don't think TSR had an official one. They tried various things, saw what worked and didn't, and repeated what worked. Gary was definitely not a fan of having many splat books, he said it many times, and in several Dragon editorials.
In their 2E days there definitely was not a business plan, or if there was it sucked or was not followed. If anything it was, "Lets throw tons of stuff out there, and see if it makes us a profit." Apparently it did, for a few years at least, but it did eventually bite them back.
WOTC, in their 3E years, did a cost benefit analysis, and apparently saw that the most consistent selling, and therefore profitable, products were the rule books, and source setting books. Modules were either a loss, or not enough of a profit margin to be worth producing. I would suspect the latter, because modules can always be marketed at a profit at WOTC's scale of business.
For 4E they still saw that the "rule" books sell the best, so they came up with their new model of new core books every year, that expand upon what hasn't been covered yet, and adding in new stuff based on the new rules sets. However, they also realized that modules not only can be done profitably, but also help drive sales of the rule books. So they started off 4E with modules, and unlike in 3E, will continue with modules to not only drive sales of the original core books, but to drive sales of their successive rule books, with the new rules, classes, etc... added in.
Obviously much more than this goes into true business plans/models, but I think that I have covered the basics of what their business plan built on.