On Amiga it was the only one of significance, as the API language. Having Lattice helped of course. Pretty much everything else was a toy in the early years.
Intuition was all C, only DOS was BCPL, which made it a pain to do anything with. AMOS was one of the toys I mention - mainly popular for hobbyists and some bad released games (it never interfaced with the OS, being ST derived. That led to its many compatibilty issues). Most games from the houses I knew, or knew people at, were either a mix of C + 680x0 or pure assembly. In commercial software (GUI based) C was the vast majority.
On MSDOS I never said it was the only choice - there were many choices, but most commercial development seemed focussed on C. DB work often ended up on Foxbase or Clipper. Turbo Pascal and C were hugely successful but didn't catch MS C. Somewhat surprising given how slow early MS compilers were.
Of course if you were doing DOS TSRs or games you'd be much more likely to use assembly in the mix.
MS C compilers not only were worse than Borland ones, although they were the platform owner, their C compiler was the last MS-DOS C compiler to get a C++ cousin.
Sadly the way Borland managed the company, let to us having to move to VC++ with MFC, instead of BC++ with OWL or C++Builder and VCL.
Only now VC++ is catching up with C++ Builder for UWP apps.
On MS-DOS besides the DB stuff, everyone I knew was either using Assembly, or a mix of Turbo Pascal with inline Assembly.
C and C++ only came into play on last high school year, just before getting into the university, but the majority of us already had almost a decade of coding experience by then.
The really sad part is MS licensed Lattice for the first couple of versions of MS C, yet Lattice itself was markedly faster. If I remember right it didn't even come with any debugger though Lattice did (not sure about v1).
I took a real dislike to Windows and MFC and moved back to the Unix side of things, so my Win programming was pleaingly brief. :)
Your experience is almost the inverse of mine - we had a few juniors comng on with Pascal as they'd learnt that in uni, but they were easy to convert to C. Just about everyone I knew in those days were C/nix or C/DOS, with just a few hanging on still trying to make a living on the Amiga - mainly games devs.
Yeah, I guess in the old days before the Internet and with expensive BBS connections, the technology had more silos than nowadays, because it was harder to move masses for any given technology.
Most significant DOS development seemed to be C.