The hardest part of investing is knowing when to sell a stock. When you do buy one, always have an exit strategy—not for right away, this quarter, this year or even the next few years. But constantly question how soon the seeds of destruction will bloom at your company. I guarantee they’ve been planted already.

Let’s probe Apple , Facebook , Amazon and Google, because they make up about 12% of the S&P 500—while representing almost all of the stock-market’s gains this year. Yes, calls to break them up persist. But that actually could increase their enterprise value based on the sum of the parts. Beyond rambunctious regulators, what could go wrong?

Apple’s problems already have become visible. Smartphones are now like radial tires. Everyone has one and they don’t wear out. Phone franchises are fickle. Ask Motorola or Nokia , if you can find them. One near-term sign of distress: Marketing tech products with splashy colors, as Steve Jobs did with tangerine iMacs almost 20 years ago, means the fun part is almost over. Apple hopes to make it up in services, but Google leads in maps, Netflix in video, and Uber in transportation. Apple is falling behind in most other growth segments. The company’s destructive seed is its desperate need for a new product category. It won’t be watches.

You might think Facebook’s biggest problems are privacy concerns, which have shown up everywhere but the company’s finances. The media cares about privacy. Users? Not so much. Facebook stock is hitting all-time highs. I think Facebook’s bad seed is population. The site has 2.2 billion monthly active users, a number that has roughly doubled in the past five years. Yet the growth rate is necessarily slowing. User count probably won’t double again, and most growth comes from the less wealthy developing world. Investors won’t like it when growth stalls.

Amazon’s biggest benefit, and problem, is its hall pass from Wall Street. It’s allowed to show very little in profits while inventing the future. That’s great, but the business of selling stuff—that is, the company’s main business—is notoriously low-margin. Retail stores never had high margins. Buying Whole Foods was fascinating, but supermarkets often squeeze out profits of only 1% of sales. Now Amazon is bursting into the health-care industry with its purchase of online pharmacy PillPack—right into the teeth of regulators. Eventually Wall Street will demand high margins after years of investment. I wouldn’t want to be around when that happens.