Why not keep the feature?
On the latest version of iOS serving up a CSV file will open in the browser as a table view, but offer up the user "More" options to save/share/air drop etc. But if the user has Apple's "Numbers" app (iOS 10+) it also gives an "Open in Numbers" option (the iOS equivalent of Excel)
I personally hate it when the mobile version of a website has dumbed down functionality and/or I have to switch to the full site just to make it useful.
For devices that handle this nicely and have a real file system (Android, BlackBerry, Windows, FirefoxOS...) they get the desired functionality out of the box. For iOS the user (depending on exact scenario/file type ) can "share" the file to DropBox or the cloud storage option of their choice, then download/view from there as and when desired.
Keep in mind that the user very likely works across multiple devices. I may trigger the download of files from my phone while browsing to work on them later on my laptop/tablet.