You'd be amazed how much this works in other life aspects.

Me too! Also, Dosbox can be simply called with the -conf parameter pointing to the desired file.
We could just have 1 Dosbox with multiple game configs!

Not really, GOG packages their games with a few different versions of DOSbox, so we could have something like that but not with just a single DOSbox executable.

Well ok, we also have that problem of the outdated versions.
Also, DOSbox 0.75 hasn't been completed since years. O_o

There is a reason GOG uses different DOSBox versions for different games though, compatibility changes. Game A may require 0.74, while perhaps Game B worked with 0.73 but won't with 0.74.

Well, it's three years later now and most people don't seem to care about DOS games anymore. I still want to mention, that this thread wasn't about reaching perfect results, but for advocating reasonable defaults, which work everywhere. (!) Outputting 856x400 to a monitor isn't going to work on most platforms. Rendering into a 960x600 viewport might work for some people and not for others - it looks awful on 1280x720 or 1366x768 displays for example.

Alternatives like the OpenGL renderer are not really optimal, because OpenGL isn't really well supported by Windows and macOS.

My solution proposed in the OP is still universal, works for every game on every platform with every source (game) and target (desktop) screen resolution at the cost of a slightly smoothed image. (And if you don't like that default for certain, you can still choose another scaler.)

Claiming "universal" compatibility is pretty bold but if you assume GOG only supports Windows 7+, MacOS, Linux (whatever version\distro they want to support) then you might be able to get away with it. GOG isn't going to do much to go against DOSBox defaults (except when they do for odd reasons....) and the DOSBox devs are pretty strict about maintaining host compatibility which is why the defaults are the way they are.

Your post isn't some massive revelation that the DOSBox devs don't know about and you claiming "universal" compatibility is ludicrous at best especially when DOSBox supports more than just the latest versions of Windows and MacOS and yes even CRT displays and who knows what aliens in another galaxy use.....

For GOG, it might be somewhat possible.

There are some tricky points between fullscreen and windowed mode especially on systems with display scaling not at 100%
and when taking in account 4k screens (and older systems)

If you focus only at fullscreen (as gog defaults to), things get easier.

True, with the more recent changes to GOG, I lost interest in the platform and got my good (old and new) games elsewhere in the meantime.

In fact I just got back to my old threads, because I has lured here by a free game on the front page. Though I don't see myself spending anything substantial, as my DOS library is mostly complete now.

DOSBox's initial and primary focus was in fact the Linux platform and their defaults are tailored towards being failsafe, because accelerated options (Overlay, GL, etc.) are not available on every hardware there. The surface renderer works on every X server, so they go with that.

However Windows and macOS have minimum requirements for graphic drivers including hardware acceleration. For current Windows platforms that means at least DirectX9 support is mandatory. OTOH the DOSBox default renderer using an outdated API isn't supported anymore.