Organizations like Facebook, Amazon, Netflix, Cloudflare, and many, many others have been combining specialized data stores for many years. Stonebraker, Madden, and colleagues summarized the argument for this approach in 2007. [0] At this point I can't think of any major SaaS application I've seen recently that did not have multiple data store types. That includes event streams, which are data stores as well.
[0] http://nms.csail.mit.edu/~stavros/pubs/hstore.pdf
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