Here's another way that you're not Dropbox (unless you are): you are not a public company that gets points from investors for removing [$35M] dollars from COGS (cost of goods sold), even if you spend [$40M] on R&D + capitalized expenditures to replace it.

Stories like this can be really misleading because large companies almost can't help but trick themselves into financial shenanigans. It's very possible that Dropbox just spent $100M upfront to start saving $35M / year relative to AWS current prices... but discovers in year 3 that they are no longer saving money because AWS has reduced prices via their own R&D, which is spread over a massively larger customer base.

Now, because Dropbox is a storage company, it may well make since for them to continue R&D and try to keep pace with AWS, at least for their own needs. But 99% of other companies - of any size - would likely fall on the wrong side of the scenario above.