Author forgot endless maintenance and peopel costs
All that stuff that apparently is a doddle to stitch together - keep stitching , because its a never ending job of stumbling over bugs, applying patches and updates, creating enhancements, and working out what root cause in a morass of interconnected custom stuff broke the system.
I know a LARGE company that thinks they can build their own cloud infrastructure because (apparently) the others arent doing it right. So far, 18 months in and circa 50 people (this aint no tinpot org) they've achieved zilch, zero, nada in terms of actual computing infrastructure their users can use. Who are instead just buying stuff from AWS and Azure that, well, it just works and is here now, not in 'alpha stage one in 3 montsh time' (its always in 3 months time). Note, this is not on premise implementation they are doing, they are setting out to build a full generic cloud with all the charging, the scaleability, the fault tolerance and so on.
They dont and never will have, the economies of scale and the deep technical expertise, nor the clout that a large cloud vendor has to batter errant suppliers into submission. They'll see this eventually once they realise that several tens of millions of pounds on people to implement "free" software, is more expensive than just buying it off the shelf. Especially since they will never likely ever release anything usable and they'll have spaffed it all away.
AFAIK none of the people in charge of this grandiose vanity project build their own cars or houses, they just buy a ready built one, yet if its computing, apparently its different.