Thank you so much for that detailed reply, really appreciate it.
So if I just send my current library to the cloud (after upgrading my icloud storage) what would happen if the MBP died? I was under the impression you needed a local copy too? Might be better for me to copy the library to an external and then sync with icloud? Although I'm sure I read some people were having problems trying to get photos to the cloud from an external disc in previous research I did, is that still the case?
If you're worried that the death of your MBP seems imminent, regardless of anything else, setup a Time Machine backup to an external drive (if you're not already doing so).
Once your initial upload to iCloud is done, your local MBP is free to expire. A local copy is not required. The contents of a new Mac/iPhone/iPad Photos library would be copied from the cloud-based library. From then on, your Mac/iPhone/iPad maintains a local library for convenience, not necessity. It allows you to work offline, and can reduce the amount of internet bandwidth required (a serious consideration if you pay your cell phone provider for data). If you have enough disk space (either internal or external) to keep full-quality copies in the local library (the alternative to "Optimize Storage"), then you can consider the local library to be a backup of the cloud.
So yes, you could first copy the library from your MBP to external, connect the external to a new Mac, and then upload to iCloud. The library on the external would then continue as the library for your new Mac. Presumably, you'd get a large enough external that your library could continue to grow, and that you could keep full quality copies locally, rather than use the Optimize Storage feature. That would give you a form of backup. However, if you'd prefer to not tether yourself permanently to an externally-stored library, then I'd suggest uploading from your MBP and maintaining an Optimized Storage library on your new Mac's internal drive.
A central feature of any "cloud" (or network server) environment is that the local machine ("client") is expendable - if the client dies or is replaced, a new client is loaded with the cloud/server-based data. All data is kept on the server, the operators of the server are tasked with ensuring the server-based storage is reliable (it's easier to do that for a relative handful of servers than a multitude of clients). In cloud systems, data redundancy is a critical consideration - techniques like RAID are used to ensure data redundancy within a data center, and servers in one data center are generally "mirrored" elsewhere, to avoid risk from local disaster.
The client can manipulate the contents of the server - if you edit a photo, or explicitly delete it, then that action is duplicated on the server and any other clients connected to it. However, the
disappearance of a Mac or iPhone does not affect the contents of the server - the cloud persists as if the Mac had never been there.
You probably did encounter some people with large libraries who had difficulties uploading to the cloud. While the process is designed to be tolerant of interruptions in internet connectivity, accidental disconnections of external hard drives, machines being powered-off in mid-transfer... still, stuff can happen, and large libraries can take days to transfer. Basic rule: Don't retire your Mac's library before confirming that everything has been uploaded to iCloud. If you run into issues during the upload, iCloud is a service separate from your Mac or iPhone - Apple provides free support for iCloud, regardless of the warranty status of your hardware.
