I always wondered the same thing. Back when I was a Google evangelist, Chromebooks gave me 100GB and 1TB for free, I got 96GB free lifetime because I was an early Google Storage paid user. Always wondered why they didn't just do free unlimited storage.
OneDrive went unlimited storage and found out the hard way when people started backing up universities of computers, tarabytes of ripped videos, and what not. They quickly went back to 1TB. This is my only guess as to why unlimited.
But why not 100GB free? My only guess is that MOST people I know don't even fill their 15GB free Google Drive storage. So Google charges a tiny bit for the extra storage.
I used to trust Google - they do a lot of good for the world (education, renewable energy, their giving app where you can donate to tons of charities for $0 overhead cost, etc...). But once they got rid of the "Do No Evil" and started doing things on Youtube I didn't agree with... decided it was time to rip Google out of my life. Wasn't easy... Took over a year and MANY failed attempts to leave. Now, Youtube, Google Photos, and gmail are the only google products I use if I have to.
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I don't know what the anger is against the 5GB. It provides a "taste" of what iCloud is like, allows users to "test it" without signing up for anything permanently like most companies do when you want to "trial" test their products. The argument that Apple should be providing more storage for free is laughable imo. Would it be nice? Sure, but they're in the business of making $.
I'd be using iCloud primarily right now if it had selective sync. Because it doesn't, OneDrive is my primary cloud provider. I just wish OneDrive backed up my live photos

. Microsoft's products are amazingly well made on Mac OS and OneDrive utilizes my 97/97 FIOS connection very well.