Naw, because using that logic when an astrally projecting Mage is killed then he should simply be "disrupted" and sent back to his body until his buddies get the smelling salts.

Even extra mellow Ravor (Yep a certain pretty young lady and I are set to move beyond merely flirting back and forth at her work, or would be if there weren't a few minor issues that need dealt with first, oh well, tis only a few short months.) has a problem with your purposed compromise as it isn't really an issue for a mage to simply reactivate said focus and at that point you might as well declare them as unattackable in the first place.

Spirits are not native, nor are they really made up of material from our space... they use magic to assume a form that can interact with our reality, but in the end, they are from a different reality, they cannot be killed in our reality, and so are sent back to their own reality (Where they CAN be killed if you so chose to pursue it)...

Disrupting a Foci (pushing it through an astral barrier or using the spell in the Digital Grimoire) is very different from actaully "Damaging" its intrinsic aura... that is what a Damaging Spell would do, Damage it's aura... now foci are pretty resilient, if you cannot destroy it in a pass, it regenerates to full capabilities/"Health" track... and I have even seen instances in games where the GM assumed that the magic actually fought back (it did so in SR2), so the attack roll was opposed, much like counterstrike worked in SR2/3... whomever won scored damage against the other... I actually liked this as it made destroying Foci somehwat of a challenge, and had the potential to actually hurt/kill the attacker if they misjudged the foci's power level...

as I said, not everyone likes this option...

Keep the Faith

Ok, I decided that I wasn't feeling quite that lazy after all, so here's the skinny as I understand it after doing a quick skim, the section on Mana Barriers are rather vague by itself, but it does specifically say that with the exception of regenning Mana Barriers are to be treated as normal physical Barriers are so I would guess that means we are supposed to use the same rules to destroy them and once destroyed they are gone forever.

And to tie it back to the topic on hand it seems to me that using the same rules should make foci tough enough that actually destroying one should be relatively rare but possible if a Mage is unable to defend against Astral Threats.

Sounds reasonable to me...