I'll chip in with a few bits.
I always (at least intend) monitor 121.5, or try if we only have 2 boxes available. I find I regularly have to reselect 121.5 after my colleague has taken the weather.
Very recently, with Brest, we were contacted on 121.5 due lack of contact. We weren't immediately aware of the silence(?) on box 1 due to a stupidly high amount of chatter and rubbish going on 121.5. Box 1 had failed, but as mentioned, there was so much chatter on 121.5 (PA's, handling agent calls, hello Bob are you there?) that the problem was masked.
When we heard our callsign on 121.5, the transmission was stepped on by another aircraft. I called the Brest channel, found a problem. realised 121.5 were calling, called them, by which time the skipper had made contact on box 1 with ATC. Between us, 2 flightdeck + ATC, we were out of contact less than 3 minutes. 121.5 seems particularly active down that way sometimes, or Is it me?
Our Box 1 had a recurring fault, cleared in the tech log.
The point I am almost making is...
Situations will still arise were people are out of contact. In todays hightened alert, the results are more costly.
Communications failures will still arise. ATC will forget to transfer. we will select the wrong freq, radios will fail. monitoring 121.5 won't always help the problem immediately.