"Would you really want a hospital where the many different machines ran many different OS's?"
A question that was effectively answered back in the 1980's - remember a basic premise of Open Systems was Standard interfaces and file formats, hence the importance of full stack Open Systems Interconnect and the various profiles (eg. GOSIP, MAP/TOP) that arose to solve real-world problems of connecting systems from a comparatively large selection of vendors, each with their own (predominantly proprietary) OS and machine architecture.
There really is no real reason why my phone needs to run the same OS, as my tablet, as my PC - in fact that is the case today: Android, iOS and Windows respectively. Similarly, I don't need desktop Word for example on a sub 8-inch screen tablet/phone, it isn't usable.
Likewise in hospitals does the MRI scanner really need to run the same OS as the bedside vital functions monitor? No they don't, they simply need to be able to use Standard protocols and interfaces such as the UK 3-pin mains power supply and either an RJ45 or multimode SC fiber port LAN connection.
In fact it is useful to compare and contrast the WannaCrypt attack with Richard Morris's Internet Worm (1988), to better understand why the widespread usage of a single OS, CPU and hardware platform (ie. Windows and the x86 PC architecture) is not a good thing.