Wdym? If a drug like this were successful, everybody with any markers for Alzheimer's would want to be in on it. They'd make bank for several years, until the patent expires, and then they'd have other methods of extending the patent and prolonging market exclusivity.
Sorry but this is a shit talking point. What drug companies fear is the eradication of an ailment, and particularly by non-pharmaceutical means (i.e. gene therapy).
Even something that's 100% effective in preventing Alzheimer's, would still see a hell of a market.
And if such a drug is possible, then there's the game theory question. Any pharma exec knows that if their company can make a breakthrough medicine...so too could another company. Is it worth it for them to sit on it, only for another company to make the same discovery a year or two later and bring it to market?