Anything given for "free" can be later taken away or reduced to almost unusable with a paid "premium" option. So you are essentially forced to pay in order to continue the service you have come to depend on.
An alternative form of payment is being subjected to data mining and classification, with tracking all over the Internet. The metadata in your photos is a no-brainer and the content of many photos can be identified with a bit of image processing. This information is sold to others or used to promote products to you.
I am using Google Photos, but it has its weaknesses, and Google does not really hide that every interaction with its services may result in something shared with it's thousands of business partners since that is it's primary business.