The surprise announcement that the telecom giant’s CEO will retire on July 1 is the latest and most dramatic example of events that have not been going as planned at AT&T. Six months ago the company announced that Stephenson would remain in his job “through at least 2020.”

Scratch that.

Last summer the company said its HBO Max streaming service would launch in last year’s fourth quarter, but it still hasn’t launched and is now scheduled to debut on May 27. Most important, Stephenson’s transformation of AT&T into an entertainment and distribution colossus has not thrilled investors as it was supposed to. One specific investor, the activist investing firm Elliott Management, which owns about 1% of AT&T, is clearly not on board.

Give Stephenson credit for boldness. He foresaw that delivering video would become the No. 1 use of wireless and wired consumer technology, which meant, he told Fortune last year, that “controlling your destiny to some degree would be really important—that is, owning premium content.” So in 2015 he bought DirecTV for $67 billion because it owned the rights to carry a great deal of programming, and in 2016 he announced a deal to buy Time Warner for $109 billion because it owned HBO, Warner Bros., and the Turner cable networks (CNN, TNT, TBS, and others).

With a few smaller acquisitions as well, Stephenson achieved his strategic vision—and made AT&T the most heavily indebted nonfinancial company in America. Total obligations exceeded $200 billion. Moody’s downgraded the company’s debt rating to two notches above junk.

Even that was manageable. The real problem was that many investors weren’t buying the strategy. They figured AT&T couldn’t outspend Netflix and Disney on content and couldn’t beat Verizon on 5G, the superfast successor to today’s wireless networks. Perhaps inevitably, Elliott Management came calling, loudly, last September. In a searing 23-page letter to Stephenson, it trashed his management of the business. His grand strategy was fundamentally flawed, the firm said bluntly: “AT&T has yet to articulate a clear strategic rationale for why AT&T needs to own Time Warner.” Elliott noted how badly AT&T stock had underperformed the market on Stephenson’s watch, then asked pointedly, “Does AT&T have the right mix of leadership at the Company?” You don’t ask a question like that unless you think the answer is no.

And even that would have been manageable if investors had shrugged off Elliott’s letter. But they didn’t. On the morning it was released, they bid up AT&T’s value by $12 billion within minutes of the opening bell.

From that moment it was clear that things would change at AT&T. In announcing Stephenson’s early departure, the company said he’ll be succeeded by COO John Stankey, 57, a 35-year AT&T man who is overseeing Warner Media until May 1. Elliott had previously signaled that it was no fan of Stankey, but partner Jesse Cohn said in a statement on Friday that “Elliott supports John Stankey as AT&T’s next CEO.” He also made clear that Stankey probably wouldn’t have been chosen without Elliott’s approval, saying, “We have been engaged with the company throughout the search process.”

With Stephenson gone, the big question is how much of his mega-strategy will go. He is scheduled to remain executive chairman until year-end, but Stankey will not succeed him as chairman, a power shift toward the board and presumably toward Elliott.

Stephenson’s career-defining bet could yet be vindicated. If HBO Max blows the doors off and customers stampede to AT&T wireless service so they can get exclusive Warner Media content, he’ll be a hero—and investors will be very surprised. For now, the unavoidable fact is that to this day the stock has never regained its price from just before AT&T announced it would buy Time Warner in 2016, when it was around $43, though the S&P 500 is up some 30% since then; the stock was at $35 in early February before the coronavirus pushed it below $30. That looks like the fundamental reason Stephenson is leaving early.

More must-read tech coverage from Fortune:

—How a pharmacy delivery startup has capitalized on the coronavirus pandemic
—SBA website leaks personal data of 8,000 small-business loan applicants
—Is A.I. better at diagnosing illnesses than doctors? Don’t believe all the hype
—What Seattle and San Francisco can teach us about mitigating the scourge of COVID-19
—Listen to Leadership Next, a Fortune podcast examining the evolving role of CEO
—WATCH: Best earbuds in 2020: Apple AirPods Pro Vs. Sony WF-1000XM3

Catch up with Data Sheet, Fortune’s daily digest on the business of tech.