Punkalouver :
it is common to turn our transponder(TCAS) to TA only after an engine failure due to performance limitations.
This is new to me, and a bit surprising coming from you, as you referred to the Eurocontrol ACAS bulletins that has as first item in their do and don't list the following :
To maximise the safety benefits and operational compatibility with ATC,ten fundamental dos and don’ts must be observed:
1- TCAS II must be operated in RA mode to provide full safety benefits
That aside , I find it a pity that you do not want to engage in the debate. The point many of us have ( and I am not working for or with PBL) is that the system is still far from perfect and even if you follow the rules , it will not protect you at all times. The GOL and DHL crew if they were still alive would sadly confirm this.
The sense reversal logic currently does not work and could induce a collision while trying to prevent one.
The interaction between human-ATC and automation -TCAS is still unclear for many , especially when ATC acts before the RA, and following the RA will mean acting against the current ATC clearance.
For those 2 reasons alone, following the RA is no guarantee that a collision will be prevented, because it needs 2 willing partners and you do not know what the other is doing.
Following your argumentation , would you be for the coupling of TCAS to the auto-pilot ? That would solve many problems wouldn't it ?