Help E: Failed to mount /cache (Invalid Argument)

[Prikers]

My Nexus 5 just failed yesterday. In recovery mode I get this message:

Supported API: 3
E: Failed to mount /cache (Invalid argument)

I followed this tutorial step by step by whenever I try to "flashboot flash" something (cache, system, etc.) I get this error message:

FAILED (remote: flash write failure)

I can confirm the bootloader is unlocked:

fastboot oem unlock
>> unlocked: yes
>> finished. total time: -0.000s

More details:

fastboot getvar all
(bootloader) max-download-size:         0x40000000
(bootloader) version-hardware: rev_11
(bootloader) version-baseband: M8974A-2.0.50.2.30
(bootloader) version-bootloader: HHZ20h
(bootloader) version-cdma: N/A
(bootloader) variant: hammerhead D821(H) 16GB
(bootloader) serialno: (not displayed in the forum)
(bootloader) carrier: None
(bootloader) secure-boot: yes
(bootloader) unlocked: yes
(bootloader) product: hammerhead
(bootloader) partition-size:aboot: 80000
(bootloader) partition-type:aboot: emmc
(bootloader) partition-size:boot: 1600000
(bootloader) partition-type:boot: emmc
(bootloader) partition-size:recovery: 1600000
(bootloader) partition-type:recovery: emmc
(bootloader) partition-size:system: 40000000
(bootloader) partition-type:system: ext4
(bootloader) partition-size:userdata: 3321fa800
(bootloader) partition-type:userdata: ext4
(bootloader) partition-size:cache: 2bc00000
(bootloader) partition-type:cache: ext4
(bootloader) partition-size:persist: 1000000
(bootloader) partition-type:persist: ext4
all:
finished. total time: 0.037s

Following this thread I suspected a problem of size so tried to increase max size:

fastboot -S 2048M flash system system.img
erasing 'system'...
OKAY [  0.126s]
sending 'system' (1020657 KB)...
OKAY [ 32.239s]
writing 'system'...
FAILED (remote: flash write failure)
finished. total time: 32.537s

fastboot -S 2048M flash cache cache.img
erasing 'cache'...
FAILED (remote: failed to erase partition)
finished. total time: 0.217s

fastboot -S 2048M flash recovery recovery.img
sending 'recovery' (10014 KB)...
OKAY [  0.521s]
writing 'recovery'...
FAILED (remote: flash write failure)
finished. total time: 0.708s

I am now clueless... This is a 5 year-old phone so it could possibly be a hardware failure but I really really want to make sure I tried everything I could to save the guy!! (even though data is long dead by now).
Any thoughts?