At the time the original 5150 was released, there were already other 8088 and 8086 systems on the market. And it didn't really strain the envelope-- no IBM-exclusive chips, and the whole 8-bit bus and support chips angle.
It undoubtedly succeeded in large part because it was a "known quantity" for commercial customers-- an approved vendor, known support and warranty policies, too big to fail. I know even as late as the mid-80s, Commodore was still advertising "You're paying $$$ more (for a PCjr instead of a 64) because the box says IBM on it"
But I was curious if there was anything that it also offered that was uniquely compelling in the at the moment of launch.
There are a few things I can think of, but I'm a little skeptical of most of them:
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The monochrome display (5151) was very well-regarded; 80x25 of very legible text and a nice long-persistence phosphor. I had one for a while in the 90s and it was quite good even though the geometry was shot. But was it much better than other "professional" machines, particularly ones using dedicated terminals or custom monitors which might also offer better tubes/drive circuitry than a repurposed home TV?
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Offering it as a turnkey package-- there were 8086 S-100 or similar setups far more robust than any 5150, but you were typically assembling it yourself, or relying on a much smaller vendor (i. e. Cromemco) to build a package deal.
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The overall ergonomic package-- I feel like there weren't too many pre-1981 machines that match the overall layout of "modest size, all-inclusive desktop box you can use as a monitor riser, and quality detachable keyboard" A backplane box and seperate drive enclosures would start to get bulky, and keyboard-is-the-case seemed to become a signature of low-end home computers.
If you walked into a brand-neutral shop in late 1981, what was the unique selling proposition for the IBM PC? The Apple II was biggest software/installed base, the Atari 800 had the best graphics, CP/M machines had established business software already.
I think you head the nail on the head when you said there were no IBM-exclusive chips. I think the openness of the system followed by the proliferation of clones drew a ton of companies and developers to the platform.
The IBM also looked like a business machine whereas others felt cheap. It may not have looked like much on first glance but they struck a good midpoint between the C64/Apple etc and the business-oriented minicomputers on the higher end of the scale.
I think the success of the 5150 was less about what it did and more about what it was and what that represented, particularly to businesses
I think in particular the part about feeling cheap is relevant. Today the plastic shell of the 64 is charming. But back in the day, that was up against the massive metal case of the IBM systems -- still legendary for weight and overall build.