So users are booted without nomodeset (which can break the setup for some appartently, e.g. my 5700XT didn't like booting without it (?)) by default, but can choose to boot with it enabled easily.
Yup, I think it only adds nomodeset to the kernel cmdline (and defaults to the vesa gpu driver, but I don't have a Ubuntu install medium at hand to verify that).
I definitely think offering the option is important and that it's easy to access, since one probably needs to use it with older install mediums when running newer hardware (e.g. when using new AMD GPUs with kernels that don't support them yet).
if you load kernel externally with -kernel, then you can just put nomodeset in -append flags. I don't see why serial console would be worse, except that for some reason inittab doesn't enable ttyS0 by default? 115200 baud is pretty slow, but I think VGA output is not much better. virtconsole is much faster but I think also requires guest modification (hvc0?).
you never know what people have connected to their serial port, so I don't think its a good idea to enable it by default, i mean in case someone boots alpine as rescue usb on some computer in a spaceship or similar. (space-x do use musl in space they say)
I suppose hvc0 enabled by default should be safe, at least for -virt and sounds like a good idea.