Unfortunately, it is a promise that has no value. With the kinds of exit we're talking about, the promise is readily broken, and with little or no adverse consequences for the start-up.

Meanwhile, there may be strong incentives for a small business to pivot away to a different strategy or to take a lucrative exit, even at the expensive of existing customers. This situation is magnified dramatically as soon as any serious outside investment is involved.

You said it yourself, right at the end, if you just consider something you wrote from the other side of the fence:

Large vendors succeed by executing to a known plan.

Large organisations in general like executing to known plans, and relying on a start-up almost inevitably brings a much higher risk of having to change those plans than relying on more stable, established suppliers.

And while small organisations might be more nimble when it comes to changing plans, stability in the supply chain is possibly even more valuable in that context, because you have limited resources. Integrating with any external dependency takes work, and if you're going to do it, you really only want to do it once and then not worry about it.

The kind of lean, MVP-producing, pivot-happy, investor-funded, five-minute-old B2B tech startups we often discuss on HN are the antithesis of that stability. The really good ones offer enough advantage over doing things another way that using them is still justified, but most simply don't and it's hardly surprising that a lot of potential customers shy away from them and head for more stable ground even if it comes at a somewhat higher price.