Could you elaborate about what makes the key on trigger "primitive" or incorrect? What does it harm? It's the same as the amp being on with the volume set to zero...literally.
For all of this one could just buy an amp with "signal sense" and it will auto turn on when it sees voltage through the RCA cables.
I am not aware of amplifiers having signal sense on RCA level inputs to engage the amp. There would be no need for a remote wire in that case. But maybe stereo technology has surpassed me since I haven't dabbled in it for a few years which would then make me the antiquated prime donas in this case. I am only aware of speaker level inputs which allow signal sense to turn on the amplifier and that option fully depends on the amp, its not standard like RCA live level inputs. A signal sense amp with speaker level inputs would be fine. The amplifier is then triggered by the radio or audio device being turned on which then engages the amplifier to power on under a slight delay. When an amplifier is simply on all the time with key on power any surge from the audio device feeding it will be amplified and fed to the speaker. This is usually heard as a loud bump or popping sound at turn on.
In the old days of power boosters with graphic equalizers and shaft mount radios (before higher tech stuff came out) you turned on your radio then turned on your graphic booster EQ. This was to protect the speakers from getting damaged.
Later came the remote trigger on many head units (stock and after market) feature which was used before that time only to drive a retractable power antenna. Booster EQ's and other similar devices started giving way to amplifier systems which incorporated the power antenna trigger concept. That generated the "REMOTE" wire on head units. Today with better head units speaker level inputs with signal sensing to turn on the amplifier are available, and as you mentioned possibly RCA level input sensing but I am unaware of that technology, which would be a very nice asset if it exists and eliminates another fricking wire.
Damage can be done to speakers when an audio unit turns on and surges. Second just having the amplifier on even with no signal going through it generates heat in the amplifier (even a class D) even though no input signal is present. It's about protecting the investment. JL Audio is good stuff. It's not cheap stuff either. So its best to protect one's investment.
My first stereo upgrade was an 8-track player. My last stereo system was commanded by an Eclipse 8053 (16v outs) with Diamond Audio 5 series amps and components all biamped. A sealed 12" Boston acoustic generated the bass. I've since grown tired of doing stereo finding that going fast is much more fun and that today's factory systems are much better than the OEM crap that was in dashes in the late 70's - late 90's.
Hope this helps explain why simply key on power for an amp remote signal isn't a good idea. You may get away with it. But it does put speakers at risk as well as work the amplifier even when it's not being used. Heat is the enemy of electronics in every form.
Have a great weekend.