This is why I wrote Timeliner: https://github.com/mholt/timeliner - a tool to download all my content from essential cloud services (like Google Photos, etc) -- I don't like to trust the cloud as a master copy of my data.
None of the services I've been backing up (Goodreads, Trakt, DeviantArt, Tumblr) are currently covered by Timeliner, but the extra twist of assembling all your data into a single timeline sounds kinda cool, so maybe it's worth contributing a few data sources.
I've had a thought about a monthly "data preservation day" concept, (on the 11th, because: https://xkcd.com/1140/, and yes, I know: https://drhagen.com/blog/the-missing-11th-of-the-month/)
A multiplatform archival tool would be useful.