"'sitting next one internal IT guy and another 3-5 internal business people with a very deep understanding of the business processes in question and solid basic knowledge about ERP systems"'

Where do you think the customer is going to find that army of hyper competent experienced staff ideling around their premisess waiting for the day an SAP project drops?

The externals arw hired as needed, expensive of course. The latter because having good people helps in any scenario.

Yes. I have been one of those. Where do you think the externals get the expertise needed that is particular to the business? We can be professional about it and try to point out the real exceses or answer generic questions, redress and inform on specific knowledge traps or consequences of answering choices placed in front of the customer, but in the end it is still a business that is running 'right-sized' being required to drop everything for a few years to assist an IT transitioning project?

Overloading the customer is one of the simplest ways to make sure ít is not your fault when things most often go south.

Then why would any one choose to use SAP with that additional tax?

I think key part is "for duration of project". Savings, if successful, then come for the lifetime of erp.

One thing I've seen frequently is treating erp implementation as IT project. I'm a technical team member doing erp implementations for last 25 years (not sap) but I'll freely admit erp implementation is not about me or technology. It's primarily a business transformation project. You need those erp business process experts to guide and customize, AND you need thorough willingness to change your internal processes to fit the industry best practices you're buying. "successful erp project" empathically does not mean "installed it and it runs". It means thorough and detailed understanding of requirements, mapping to new processes, customization where absolutely positively necessary, substantial and organized and embraced business transformation, extensive training, and thorough testing including user acceptance testing.

If you think is erp as something you just install and life will be the same but magically better, you're gonna fail hard. Missed requirements and edge cases, and or significant internal resistance to change, are frequent challenges.

Because SAP is what most other companies in the sector use and because there are few alternatives. And because everyone else is m uses it, it must be good. Right? Right?!

Also, because leading employees usually have no idea about the intricacies of the "old" system and think it's easily replaceable with a solution from the shelf.

Becasue the alternative might even be more expensive, business wise. Also, ERP projects are not be taken lightly.

Yeah, I can't see any better argument against contracting an ERP package. This is exactly true, and those 3-5 way more expensive new developers you are hiring through a 3rd party won't create anything better than the original one that knew your company. (They are not temporary either, because the duration of the ERP project is forever.)

And the alternative to an ERP in an manufacturing environment is what exactly, in your opinion?

The alternative to an ERP package is creating software that manages it.