This is why humans invented contracts. No two businesses enter into a $1 million deal together without a contract to describe the expectations of the partnership. For this reason, no service provider should be "surprised" by the level of commitment required to fulfill its end of an enterprise deal, because all the expectations should be in the original contract (literally, the service level agreement). There are no surprises.
Also, (pedantic) in your example, $1m a year is beyond sufficient to cover the costs of two engineers and some support staff. Besides, realisically the biggest support issues will be problems with availability, which presumably will be network wide and not limited to one client. I highly doubt slack needs to hire dedicated engineers for each new enterprise client. Instead they can reallocate existing engineers when needed, and grow their total labor capacity as it becomes constrained.
Also, (pedantic) in your example, $1m a year is beyond sufficient to cover the costs of two engineers and some support staff. Besides, realisically the biggest support issues will be problems with availability, which presumably will be network wide and not limited to one client. I highly doubt slack needs to hire dedicated engineers for each new enterprise client. Instead they can reallocate existing engineers when needed, and grow their total labor capacity as it becomes constrained.