AS/400 still exists actually, under the name IBM i. I know a person who swears by them. It's a fascinating system. There are no files, only persistent objects living in a 128-bit address space. Downside: still uses EBCDICβ¦.
Actually single-level store concept was introduced in Atlas, used in MULTICS, AS/400. [1]
Some modern and quite high-performance solutions are using this concept, e.g. LMDB [2]
Not so much for "traditional" desktop apps. This sort of performance isn't going to do anything for your spreadsheets or word docs. It might pave the way for some really interesting video games though!