As far as investors can see, the outlook for Apple's shares remains as bright as an iPad screen despite the resignation of Steve Jobs, the company's legendary co-founder, as chief executive.

But many investors worry that the outlook for the medium- to long-term has become very cloudy.

Jobs exits as CEO at Apple's high, with revenues having steadily grown each quarter over recent years and analysts expecting a terrific performance in the next holiday season. Shares fell just 0.65 percent on Thursday, withstanding steep falls in the broad market.

But with many equating Jobs' vision with Apple's success, there is a fear that competition will finally gain on the company years down the road.

In the long term, if Steve Jobs' health deteriorates or if he becomes more disengaged and does not lead the strategic aspect of the company, we will probably cut back our position by half, said Channing Smith, co-manager at Capital Advisors in Tulsa, Oklahoma. Guys like Jobs don't come very often.

While Apple's product lineup should hold an edge over the competition in the next couple of years, it is the long-term outlook that has investors worried.

The impact of Steve Jobs' absence will be limited at least for the next two years because all the products that come out during this period will have his finger prints all over, said James Meyer, chief investment officer at Tower Bridge Advisors in West Conshohocken, Pennsylvania.

Even after passing the baton to Tim Cook, Apple's chief operating officer, Jobs will remain on investors' radar. Most still hope that from seat as the company's chairman he will provide guidance on key projects.

But the stock could take a dive if it becomes clear that he is no longer able to contribute to Apple's strategy.

In the long-run, considering that he is an irreplaceable icon, ... is Tim Cook the man? We don't know, Meyer said.

We're witnessing a business legend moving toward the exit door. Time only will tell if the company maintains the innovation and the creativity that he put in place there, said Keith Wirtz, chief investment officer at Fifth Third Asset Management, with $16.3 billion in assets.

Wirtz, who like many other fund managers has Apple as one of the largest holdings in its portfolio of large-cap companies -- about 6 percent -- is sticking with his existing commitment to Apple shares. He is betting that Jobs' culture will continue to inspire the company, especially if in his new role as chairman he remains involved in major development projects.

And a number of investors said they'd be more likely to buy shares if the stock stumbled.

David Rolfe, chief investment officer at Wedgewood Partners, said he was buying on the dips for his new Riverpark/Wedgewood Fund.

It just so happened that we got some cash inflows in the last 24 hours so we have been buying Apple in the fund. If the stock had been hit harder, we would have added to it in our separate accounts as well, he said.

Bank analysts overwhelmingly kept buy recommendations on the stock, with price targets ranging from $460 to $525 for the next 12 to 18 months, although many warned of increased volatility risk.

So far, selling Apple's stocks after each of Job's health scares has proved to be a bad investment decision. The stock has taken a hit right after the announcement of each of his three health-related absences, but quickly recovered.

In January 2009, the shares dropped almost 11 percent within the first week after Jobs announced his medical leave, but by end of that month they had more than recovered all their losses.

This is the lesson for the last seven and a half years because Steve Jobs has been sick or recovering or in remission for all of it: Wall Street cares less about Steve Jobs' health than it does about Apple's health and Apple is healthier than its ever been, said Stephen Coleman, founder of Daedelus Capital LLC, which manages $4 million, 75 percent of which is in Apple.

(Additional reporting by Angela Moon and David Gaffen; Editing by Leslie Adler)