For those who don't know: compare the visual design of https://www.dropbox.com/transfer with the design of https://wetransfer.com/. Design and location of key elements are super similar.
1) For a company with an established sharing model (shared folder, live-updating links), a departure or addition to this needs to be thoughtfully orchestrated. Moving to snapshotting content, gathering content inside and outside your Dropbox, and enforcing expiration in this tool is a very different paradigm.
2) However, specifically because we are also a storage provider, we can provide a much faster sending experience for them as we can remove the upload step, which is especially important for folks sending really large files. We have a large user base storing their content in Dropbox, this is a net new added value/functionality for them.
And in this case, I would still hope they did do market validation internally, even if it is just a copy of wetransfer. Just because some other company is doing something doesn't automatically mean it's a good fit for your company to do it, too.
As for the design, it's pretty hard to say that "a generic file selection panel in the middle of a page and a Sign Up button at the top right of the page where it typically is on most sites" is a "straight up clone". The color schemes are similar, but that's just because the main color branding of both companies happens to be light blue.