What happens when you run out of space in a bottle you paid for? You buy (more) bottle before you can carry more water with you.like what happens once you run out of space you paid for?
Most people don't care. "Simple and easy" consumer software does what most people need and no more...what if i do not want every damn picture to show up everywhere.
You are not using the term "backup" properly. If you upload a picture to Dropbox and delete it from your hard drive, Dropbox is not a backup. It is your primary storage.i guess its not meant to be a proper backup solution. the one reason i use services like dropbox etc is so i do not have to waste any actual space on the device itself
Similarly, if you run out of space on your device, you can choose to keep the optimized versions only in Photos and it will delete as much as you need when you need space for something else. That way you don't have to keep track of which (and how many) pictures to delete manually and remember from where to re-download them when you need them.
In addition, Photos can be your "real" backup. You can keep every picture at full size on your hard drive and have them backed up in the cloud.