TCAS is mandatory for commercial a/c but MAY BE inoperative (MEL), usually for 3-5 days. On the other hand, xponder with mode C is mandatory, as many ATCs don't have indication of primary radar return.
A/c with TCAS inop but working xponder with C mode will be seen by a/c with TCAS and even TA/RA will be issued but only to them.
Even a/c with xponder without C mode are seen by TCAS but without altitude info.
Years ago, flying late in dark,desolated, mountainous area, we tried to outclimb Venus mistaking it for the light on top of the mountain.