Topdrop, there is a good reason why ICAO requires or recommends that aircraft monitor 121.5
It is that overflying aircraft can hear a pilot transmitting on 121.5 even when ATC's ground-based stations cannot, and when aircraft tuned to those ground-based stations cannot. An aircraft at 40,000' can hear a station on the ground at a range of about 250 nautical miles (remember the old rule of d=1.25(root h) d in nm and h in feet?)
There is no "instead" about this. I asked "What reasons are there why 121.5 should not be considered, along with other frequencies, when making the decision about the appropriate frequency to monitor?"