Seeing as I haven't heard of it, it doesn't seem to meet my criteria of being popular enough to be likely to be around in 50 years.
EDIT: It's Glacier I haven't heard of, not Amazon.
EDIT: It's Glacier I haven't heard of, not Amazon.
In any case, I think the criteria of "I haven't heard of it" is a little strict, given you've made an open question asking for advice on what to use...you should be open to the possibility that there are people here with expertise who know of things that you haven't heard of. For example, you don't seem to have heard of Amazon S3, which Dropbox is based off of. Amazon is also the company behind Glacier, and while Amazon is far from being a sure bet of lasting 50 years, it's a good as bet as any company.
I don't think you can guarantee 50 years, regardless of number of services used.
But you'll have plenty of time to offload them before any of these big names shut down.
Also, I'd guess this is your best bet for > 50 years.