update-alternatives should probably not get used for packages for which
it usually makes no sense to have both installed at the same time
anyway.
Well, as a policy matter, I agree that having functionally identical packages that differ solely in the file format does not make sense ... and users who want both TTF and OTF binaries for whatever reason can still get them elsewhere and install them beneath $HOME. (Assuming that it is documented somewhere — discoverable for the user — that such an option is there.)
The trickier question is what would be the best approach if the two binary options are not functionally identical.
I can't imagine a scenario in which software would break because a font binary used quadratic Beziers versus cubic Beziers, although maybe that's possible. But you might have different hinting and features. As far as I can tell from the Source Serif binaries, the .OTF files include PostScript hints, whereas the .TTF files don't, but there definitely aren't other differences between the features or so on.
It's kind of an academic exercise for TTF/OTF, but a more pertinent question to consider for VF versus static.
Also, it's a command line tool and rather clumsy to work with,
so this adds complexity to the user.
More so than update-alternatives is for other packages? I'm not the maintainer of update-alternatives or anything, so please feel free to point out all of its shortcomings; I want to understand the side effects of different approaches here. In an ideal world, I would hope that such alternatives would be managed by a font-manager application, although it's clear that current ones don't.
And, setting it up to provide a
choice between a single font file and a set of font files is probably
also rather tricky for the maintainer.
I don't understand this; are you saying that having one source package that builds several binary packages is wrong? I actually thought that was common. Or do you mean it's bad to have source that builds into a different number of output files?
Having two packages conflict each other and having one
provide the other so dependencies are fulfilled is a lot easier to work
with in comparison.