How to disable systemd agressive emergency shell behaviour?

It is literally only mount failures, that's all you would need to change.

So the letter of your request would be trivial to answer. Create a drop-in file:

# /etc/systemd/system/local-fs.target.d/nofail.conf

# Clear OnFailure= (set it to nothing)
[Unit]
OnFailure=

I believe this will add no new problem, beyond those that linux sysvinit already suffered by allowing this partial failure scenario.

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However you also pointed out the question of how long systemd should wait for the specified block devices to become available. I can see no way to configure this, without providing a replacement for the fstab generator as a whole. https://www.freedesktop.org/software/systemd/man/systemd.generator.html

If you dump a large amount of less widely-used code here, it seems unlikely to increase system resilience. I think the closest solution would be to patch the existing fstab generator. It's not massively complex, I suspect you could get away with it / keep up with any significant changes.

Technically, if your distribution had a self-contained mountall sysvinit script, you could try hooking that in. But that will significantly change the boot process - it's actually more of a fork. I would not recommend that approach.

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https://unix.stackexchange.com/a/393711/29483

If you search through the unit files, there are only a very few ways for the boot to fall back to emergency.target. It's usually when a .mount unit for a local filesystem fails, causing local-fs.target to fail. Or when your initramfs fails to mount the root filesystem, if your initramfs uses systemd.

local-fs.target has OnFailure=emergency.target. And it gets failed because units for local filesystems are automatically added to the Requires list of local-fs.target (unless they have DefaultDependencies=no).

$ systemctl show --property Requires local-fs.target
Requires=-.mount home.mount boot.mount boot-efi.mount