This is especially true as Amazon (AMZN) Web Services continues to roll out products to compete with offerings from legacy software giants—one of the latest being a database migration tool to move Oracle (ORCL) or Microsoft (MSFT) SQL Server databases to AWS.

Even ardent AWS users, thousands of which are at AWS Re:Invent this week, do not want to repeat past mistakes of dedicating too much of their IT budget to one vendor. That “vendor lock-in” has been experienced by innumerable companies with Oracle, VMware (VMW), IBM (IBM), SAP (SAP) and Microsoft. Business customers complain privately that these vendors then use overly complex licensing agreements and software audits to wring more money out of them. Needless to say, this does not leave them feeling the love.

AWS executives use that very argument against the legacy vendors. At AWS Re-invent, Amazon senior vice president Andy Jassy said: “I haven’t met a database customer that is not looking to flee their vendor.”

“They audit their customers and fine them if they are out of compliance or negotiate with them to use more of their cloud services,” Jassy noted. That was one of the biggest applause lines at his keynote on Wednesday. Amazon’s response to this last year was Aurora, a MySQL database clone that Amazon claims offers the same performance as commercial database at one-tenth the cost. The database migration tool and a schema conversion tool, announced Wednesday, are this year’s responses.

But here’s the thing: In the cloud realm it is not Oracle or even Microsoft that is dominant. It is AWS. As Jassy reiterated during his keynote, AWS claims more than 1 million customers, is on track for a $7.3 billion run rate this year, and had 81% year over year sales growth. He also cited Gartner stats that AWS fields 10 times more computing capacity than the next 14 (!) cloud competitors combined. That’s great for AWS thus far, and perhaps good for customers fleeing legacy vendors, but also speaks to a worrisome dominance, according to several show attendees who did not want to speak for attribution given their relationships with Amazon.

A C-level exec at a large financial services company said that right now, AWS looks like a “benevolent dictator” driving the public cloud market agenda. But he worried that there’s a fine line between a benevolent dictator and a plain-old dictator.

Another attendee, an exec with an AWS vendor partner, put it this way: “People talk about lock-in with Oracle. But data gravity makes lock-in worse with Amazon,” he noted. Data gravity is a tech term that means that once data is inside a given repository, it is difficult and expensive to move it. “With Amazon, there are fees to do that and they’re hidden until you try to do it.”

Others downplayed that issue, noting that it’s fine to use basic Amazon EC2 compute and S3 storage, but things get hairier once you start adding higher level workflow, messaging, and database services. At that point, it becomes difficult to move to another provider should the need arise, they said.

In an interview with Fortune, Ariel Kelman, AWS’s vice president of worldwide marketing, said Amazon’s business model precludes the bad behavior of legacy IT vendors.

“The key thing is we optimize customer choice highly in terms of what services they use as opposed to telling them to use this new Fusion architecture and to to buy 15 things to get it working together,” he noted using Oracle’s Fusion strategy to make his point. He also noted that large customers can run MySQL databases in house and Amazon Aurora in the cloud, basically cloning their operations in both places. Then if there’s a problem they just use their on-premises operation. He cited Comcast as one Amazon customer doing just that.

In a press Q&A session, Jassy also contrasted AWS with its IT forebears. “Most of the old-guard tech companies have lost their will and DNA to invent. They acquire.”

He also said that cloud computing, by its very nature, differs from the old proprietary software era in that customers can buy resources by the hour. “We have to earn our customers’ business every hour, every week. If they want to leave they can leave,” Jassy said.

But, if customers use more than compute and storage, their engagement is less and less a pay-as-you-go model, and more a data center or server room replacement model, several attendees said. That makes picking up and leaving a tougher proposition.

Jassy also noted that 90% of AWS product decisions are based on customer demand, although as someone who’s covered Microsoft, Oracle, and other legacy companies for quite some time, I can say those companies all used (and use) the “driven by customer demand” claim.

He also noted that Amazon lacks of the bare-knuckled enterprise software sales culture that drives Oracle. AWS salespeople are not calling customers on the last day of the quarter to book a sale to put them over their quota, he noted. Okay, that’s a valid point.

So the takeaway here is that AWS is at the top of its game, even with Microsoft, Google (GOOG), and IBM (IBM) pouring billions into their respective public clouds. So it’s up to big customers to weigh their cloud buying decisions carefully with an eye to the future, but also remembering how vendor lock-in has dinged them in the past.

As AstraZenica CIO David Smoley told Fortune recently: “Vendor lock-in is a concern, it aways is. Today’s leading-edge cloud companies are tomorrow’s dinosaurs.”

Fortune has more Re:invent news here and here.

For more on Amazon Web Services, see the video.

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