iCloud Photo Library activation on my phone deleted majority of pictures I recently completed a complete upload of my photo library from my mac's photos app (whose local repository constitutes the complete set of my pictures) to iCloud photo library. Wary of future issues if I had activated iCloud photo library on my phone simultaneously. I let this upload complete before activating iCloud photo library on my iPhone. This turned out to be a well founded precaution, because after turning iCloud photo library on my phone today, about half my photos are no longer accessible on my phone, but they are still on my mac and iCloud.com. 

On my collections view in my iPhone's Photos app, I started by having many hundreds to a few thousand photos per year from 2008 to 2018 for all years. After turning on iCloud photo library, I now have no photos from 2008 or 2009, and only a handful remaining from 2010 to 2016. The photos that remain in my photos app might be only the ones that I took on my current phone (which I bought in 2016). How do I get the entire set of photos that are currently stored in iCloud photo library onto my phone again?

Edit: 
     A more specific and descriptive timeline of events below:

Let set "A" be all the photos that I want to exist on my mac, my iPhone, and iCloud. 1 week ago, my iPhone had A stored locally and iCloud was off. I then imported A from my iPhone to Photos on my mac. Now A is on my mac and my iPhone locally (iCloud library is still off). I then turned on iCloud library on on my mac and the upload occurred for about a week. As of this morning, A was in iCloud completely.

After I confirmed that, I turned on iCould library on my iPhone, a device that had A stored locally already! Despite this, about 8 years worth of photos (2008-2016) are not accessible on my phone anymore! There should be no deletion / downloading between iCloud and my Phone because the set of pictures in both places was the same!
I'm adding a summary of the discussions the OP and I had when trying to troubleshoot this issue.

This issue was due to how the OP copied the original photos: iPhone -> Mac -> iCloud.  The photos remained on the iPhone and the assumption was that iCloud would not have to do much work at all.  The exact same photos that were uploaded and sitting in iCloud were sitting on the iPhone. 

The photos first had to be synced to iCloud from the Mac. With the large number of photos involved, that took several hours to complete. 

After that, iCloud then begins to sync the photos back down to the other devices signed-in with that same Apple ID, in this case it was the same iPhone.

The iPhone will only perform iCloud photo syncs when the device is on wifi and plugged-in to power (with iOS 11, you have the option to allow syncing on cellular connection via Settings -&gt; Photos -&gt; Cellular Data and turning on Cellular Data).

iCloud took over 9 hours to sync the photos back down to the iPhone, replacing all the exact same photos that were already stored there.

To avoid this, sync the photos from the iPhone -> iCloud first.  After that, it will sync down to the other devices, like the Mac.  It will still take several hours depending on how many photos are being synced.