If there was healthy competition that same number of jobs may be split over multiple companies, but it would still have to exist since the market for the product itself wouldn't change.
That isn't an argument for not breaking up a monopoly. Only an argument that the market may be big enough to support actual competition.
What is being claimed is that the majority of those employees are not seeing an increased benefit (wages) proportional to the increased profits of the company they work for, whereas shareholders and executives do.
If it were multiple competing companies, the same output would probably require MORE employees. One of the ways it is so profitable is to implement economies of scale not possible in a less monolithic industry.
You are failing to see that extractive business models such as monopolies do not add value, they extract value and concentrate it in the hands of the execs/owners. This removes capital from the socio-economic system/society and reduces the capabilities of the whole system to grow (the occasional purchase of a yacht does not rebalance things).
That isn't an argument for not breaking up a monopoly. Only an argument that the market may be big enough to support actual competition.