However defibrillator implements the same mechanics. That item is the only that allows to resurrect players. And it takes first aid kit slot.

You raise a very fair point of comparison; defibrillators do serve a utility that no other item provides. Though defibrillators are often derided for good reason: rescue closets.

Not only do they fulfil the same role but can return multiple teammates at no cost of an item slot. They're littered throughout all maps (especially after mid-level panic events), and respawn timers are ludicrously short. Versus and realism both disable rescue closets (thankfully), which is where I think defibs fare better. But even then, I often see players resort to using defibs more like first aid kits with greater set-up. Intentionally killing a survivor to momentarily bring them back up to 50hp.

The defibrillator remains an awkward item, balance wise. Its role to resurrect dead teammates is overshadowed by its more useful capacity to heal. And I think players tolerate this because the tactical decision of WHEN to execute this risky heal manoeuvre beforehand, is more rewarding as a preventative measure than waiting around for someone to organically die; like Valve had anticipated.

I say all this as someone who actually enjoys the 1 defibrillator + 3 first aid kits composition: I can't speak for others, but I do suspect most players overlook the item because their intended use is very restrictive. Similar to how I think adrenaline would suffer, if reworked as the only way to revive incapacitated survivors. John Guthrie makes a really good point when discussing their implementation of adrenaline in Left 4 Dead 2's developer's commentary:

Originally posted by John Guthrie:

"replacing the pain pill item was a challenge. Initial versions of Adrenaline allowed players to run faster, and perform certain actions faster, such as reviving other players. But the pills already give 50 temporary health points, which is enough health for injured players to run to full speed again, so the adrenaline needed something extra. We expanded the actions that could be affected by adrenaline [...] Although it doesn't give as much health as the pain pills, we found many interesting strategies began to emerge from the well-timed use of adrenaline."

Expanding the item's potential is a far more agreeable approach than stripping mechanics away in order to force players to rely on said item. I'm grateful you did mention defibrillators though... it's given me a fair bit to mull over. And I think this discussion has now provided me with some new ideas, I to want further explore concerning both the defibrillator and adrenaline.