Interesting points.
I will try to answer also point by point.
First of all,yes you can do bends and stuff with guitar synths. I am very happy in this regard with my triple play..thing is that almost all of the synth sounds I love and use were never "built" to be translated into string style playing. Ofcourse not. So even I can achieve results I really dig with my triple play there is always this 1% of things I cant do. For example I would go crazy if in the future some developer achieves algorithms with a guitar synth which can translate "all of it" what an electric guitar can do with a (let's say) moog lead or sax.For example pinch harmonics. How could this sound? Maybe interesting,wild,unexpected?
We dont know. Until yet nobody has ever tried this.
As for using a keyboard I can use ofcourse the stick/wheel for bendings/vibratos but come on..this is not the same..Also I cant replicate 100% the runs & phrasing I use on guitar.
Anyway..this for the "technical side of things".
So..yes "in the past" guitar synth was a niche thing. Prog rock. Right?
But in the past we did not have a "diminished interest" in rock music. Actually rock music was the "basic thing" with all it's different kinds. Guitar,Bass,Drums ..a sax here and there,with synths things became "more diverse" and indeed a lot of prog rock stuff appeared. Niche.
But is this the same today? I dont think so. Everything has completely changed. Kids like the sound of an guitar but it is just "one of many sounds" they grow up with. Cheap,nasty drum sounds,all forms and kinds of bass sounds,in general "leads" or "solos" are not a "important part of it" and are mostly replaced by a vocal hook or even a more hefty rap part.
Judging by what I see,what I experience seeing and listening to what the kids hear today..they have a completely different approach to sound.
If in the 80s heavy metal.was "extreme" and "aggressive " today kids think it is "lame" and lacks bass!Today heavy metal is a play ground for guitar players who truly to.be more like bass players with guitar skills.
Not to mention that all the gangsta & electronic music "done the rest" in the last 20 years. This is a fact.
Long story short electronic/synth sounds are not "niche" today. They have replaced most of what role had the traditional rock/pop instruments 1955-1990.
They are like 70%-80% of what we hear today. Most likely this will not change. Kids will do their "melange" they will mix stuff and sounds from 1955 until today. They will hear on YT Chuck Berry and one minute later Snoop Dog.
Dont forget that just now Kate Bush went into the charts again with a 40 years old song. Kids want it all and they can have it.
The electric guitar may become soon another trigger tool like the midi keyboard.
Edit
I forgot to mention that I most often use my fav profile and "mix" a little bit of a synth lead to it. And ofcourse if you use a piano sound doing the vibrato it will sound funny. I have to turn down the mix and in general it is often a "lots of work".
A more "intelligent" guitar synth would do this automatically?
Stuff like that..