Stops All Production
I was lucky in that I never had to deal directly with these database monoliths. I worked in QHSE, advising and auditing hundreds of businesses in the energy sector (mainly oil & gas, but also some electricity). Back in the 1990's, several of the larger corporations I dealt with were running projects to install SAP - quite a number gave up before completion and one even got their system running before giving up on it. They were sold a great idea, a way to put all necessary data in the hands of the people who needed it, to be as efficient as they could be in day-to-day operations; unfortunately, the downside was that they had to change the way they did things in order to make the new system feasible. Most organisations ran their project in stages and, whilst each stage could be made to work, the changes needed meant the previously "completed" parts had to be updated. It was a recipe for breaking every budget and target. The only operation I knew that actually got it working to their benefit was a new project where they had their own development and implementation team embedded right from the start. But, being a project, despite its success, everything was decommissioned on completion and, as far as I know, almost nothing was able to be transferred to any others.
To many, the name SAP had the meaning in the title of this post. The sweet spot with any of these ERP-style systems is implementation below any level that will be attractive to SAP/Oracle/etc...