I presume that Kleiner Perkins has by now distributed its shareholding in Google to its own investors, the university endowments and other limited partners that entrusted the Silicon Valley venture capital firm with their money. If so, this would be the perfect time for John Doerr, the legendary investor who got KP into the fabulously profitable search engine, to retire.

Doerr, 55, is a legendary investor whose career has spanned both the PC and internet booms Since he joined KP in 1980, he's spotted Compaq, Sun Microsystems, Netscape, and Amazon.com. His only big misses: Microsoft and, arguably, Yahoo. Even Doerr's most public failure — his support for a vehicle which began as a dream to revolutionize urban transport and ended up as the laughable Segway — shows a naive utopian faith which will one day be seen as charming. And investors are forgiven their occasional mistakes if they can end their careers on an investment such as Google.

Kleiner Perkins paid $12.5m for a 10% stake in the search company. Other investors couldn't persuade their partners to approve financing for a new entrant in what was seen, then, to be a played-out field, dominated by companies such as Altavista and Inktomi. And there was no revenue model. But Doerr, with Mike Moritz of Sequoia Capital, who also invested, had enough of a track record to overcome any skepticism. He had the faith of a technology enthusiast, that a better search engine would attract users, and any company with an audience could figure out a business model.

The rest is venture capital history. Google is worth, last time I looked, about $150bn. Even with dilution from stock option grants and the initial public offering, those original Kleiner Perkins shares would now be worth in excess of $10bn, a return of at least 1,000-fold on KP's original investment.

One can understand why the IPO itself might not have been a good moment for Doerr to leave. It wasn't, itself, that successful. The stock drifted for a few months after it hit the public markets. And, in any case, Kleiner Perkins only began to distribute shares to its own investors in November 2004. Typically, though it operated on an accelerated timetable in the case of Google, KP used to disburse shares in successful portfolio companies in eight tranches over eight quarters.

In any case, the Google shares are now in the hands of KP's investors and partners, or cashed in. This was John Doerr's most successful investment in a glittering career, a perfect moment to retire and pursue philanthropy or some other, public, mission. This dorky former salesman, with trademark electronic devices attached to his belt, would take his place in the pantheon of 20th century investors, and still have time to save the planet in the 21st century.

John Doerr has missed the few large opportunities of Web 2.0. Kleiner Perkins missed Skype, Myspace, Facebook and Youtube, the top four ventures to emerge over the last few years. Doerr's new interest in clean technology, to which Kleiner Perkins claims to be committing half its resources and energy, is one of two things: either a rather desperate attempt by an aging surfer to catch one last wave; or else a mission to save the planet, and secure his reputation, at least in the eye of his own, environmentalist teenage daughter.

If Doerr is indeed holding out for one last big win, he should remember the rhyming adage from the world of politics: all careers end in tears. As a venture capitalist, Doerr is extremely unlikely to top Google. If his mission is now one of good works, better to fund it personally, or with donations, than with the investment capital of Kleiner Perkins' limited partners.

The world may applaud Doerr for his deep moral commitment; KP's backers will apply harsher financial metrics of success. As an investor, the Valley VC's luster can only decline. John Doerr, for your own sake, go. Now is the moment.