Please step away from the rose-tinted glasses.
> Was the most flexible OS MS had done
Flexible in what sense? In that there wasn't a lot to it, but it let you do pretty much what you wanted because it was only a step away from- and didn't prevent access to- the bare metal?
It's not as if MS-DOS was good even when it was new. It was little more than a quick and dirty (*) 16-bit port/ripoff of CP/M that Bill Gates bought in and rebranded. CP/M was designed for incredibly limited 8080/Z-80-based systems in the mid-70s, and its limitations were understandable on *that* basis.
But even by 1981, that architecture- which QDOS/MS-DOS essentially just copied- was already unnecessarily dated and primitive for a shiny new high-end 16-bit computer.
Pretty much all the later convoluted complexity and bodges of MS-DOS were due to having to work around the limitations of the original design while retaining compatibility with it. (Well, that and the clunky Intel x86 architecture which was limited by *its* mid-70s origins and convoluted design upgrades in a similar way (**)).
People like my Dad who used PCs in the 80s/early 90s say "oh, that [i.e. the mess of MS-DOS] was just how computers were back then". No, it wasn't, it was how the crappy IBM PC design and its already-dated OS was.
MS-DOS was crap then, and it's crap now. Good riddance.
(*) Hence its original name QDOS- quick and dirty operating system.
(**) Apparently even the designers of the original IBM PC wanted to use the Motorola 68000 rather than the Intel 8088 for that reason, but were overridden by the beancounters.