It is a waste of time to install a 121.5/243 ELT. Why bother. You can get a good 406 ELT fairly inexpensively. It is
far far more accurate and more likely to save your bacon.
The new ELT's have better batteries. The old ELT designs go back to the 1970's.
ADS-B is NOT a substitute for ELT by any stretch. That is just not correct.
121.5 is for local homing and has limited range. It is not monitored by satellites. All 406 ELT's have 121.5 as well. No new ELT's have 243.
243 is monitored by military satellites as a courtesy, but coverage has huge gaps in geography or frequency of time passing overhead. Then the time to report to local authorities, lack accuracy, means no one will know who you are, where you are for a long time, if ever. For all intents and purposes 243 is not monitored.
406 for gosh sakes, I am cheap but this seems like a no brainier. If you pipe in GPS position into the 406 ELT you are far more likely to get help fast. The constellation of new 406 satellites makes the choice of what should be in your plane obvious.
At some point 406 will become mandatory.
121.5/243 Legal? OK. Personally I have a yellow EBC I use for a paper weight. What's the next question, can I install a King KX170B and NDB remove my GPS? Yes you can, but somethings need to be retired (including me, soon I hope).