jonkster
Your analysis and conclusions are pretty much spot on. I make a few points.
Monitoring 121.5 and broadcasting on it in the event of an emergency would be far less likely to achieve satisfactory action than if done on an FIA frequency. On 121.5 you would be likely to at best be heard by a high flyer, who may be an international with a poor command of the English language, or even if a local may also not have a clue what you are saying WRT your location and difficulties just when needed. In any case, they would have to note what you are saying then relay to ATC on a high level frequency and act as the go between.
However a broadcast made on an FIA would mean you are likely to be in direct contact with the ATC responsible for your area with a good knowledge of locations, and able to provide direct assistance including with other aircraft in the area. If you are below the ATCs VHF coverage, these other aircraft in the area will most likely to hear your call and will relay and are likely to have some familiarity with your location.
This has been the practice followed for at least the last 40 years or so of my experience. I've heard many Mayday calls directed to FS/ATC on FIAs, none heard at all on 121.5, only the odd ELB. Monitoring an FIA also brings all the other advantages as previously outlined in posts here.
Deleting FIA boundaries and relying instead on boxes placed in the vicinity of a communications outlet would also introduce the problems associated with not necessarily talking to the ATC responsible for the area you are in. Some outlets are not located in the area they serve, and instead may be on a mountain top outside the area with directional antennas pointing towards the area served.
As you have said, it is far easier to interpret a chart or moving map display with boundaries included.
All points made some 15 years ago and no doubt the reasoning behind the then industry associations requesting the boundaries to be reintroduced promptly to the charts.
NAS Chart simplification! why, why, WHY?
As far as I am aware, there is no industry proposal to delete FIA boundaries from charts. If there were, the proposal would need to be subject to a benefit analysis, HAZID and safety case and widespread industry support.