So if this is the case, than how do I download photos to my mac and keep them on the hard drive whether I delete photos from iPhone or iCloud?
You can always Export (File > Export). Photos includes a Recently Deleted album (images are kept for 30 days by default), which makes it easy to identify the images you might want to Export. Since there is no "Delete from iPhone" as it's really "Delete everywhere," you might want to do your routine house-cleaning while you're at the Mac. You could first Delete, then go to the Recently Deleted album to identify the items to Export. Once Exported, you could go to Recently Deleted and permanently delete the items (since, like a Trash folder, it's not really a space-savings until you permanently delete).
It might be worthwhile to look into creating an AppleScript to automatically export from that album (can't say whether or not it would work). Time Machine is another way to recover deleted items.
The concept behind Photos' Optimize iPhone Storage option is to let iOS manage space utilization on the iPhone, rather than selectively delete images on your own. If space is needed for other purposes, iOS will delete the least-viewed images from the iPhone, leaving only a thumbnail behind so you can still locate the image. If you choose to view it again, it'll be downloaded from iCloud. Whether this is more space-efficient than deleting manually is hard to say, but the convenience of having an automated process may outweigh questions of absolute space utilization.
Of course, Optimize iPhone Storage doesn't address the desire to reduce the use of paid iCloud storage - it would be nice if Apple added an option to automatically archive deleted images. Maybe Photos 2.0?
Editorial comment: There's really nothing wrong with deleting obvious junk
everywhere. Yeah, we all tend to save every image to our HDDs since they don't take up physical space, and you never know what you might want in the future. But we may want to take a page from those artists who destroy their unsuccessful works - is there a point to leaving tens of thousands of mediocre images to our grandchildren? At best, they'll go through the albums created of the better images. Pick the good ones, ditch the worst. The more junk we have lying around, the less likely it is we or anyone else will ever go back and look at it again.
