List of artists that have never lost quality throughout entire career

Of course a rock.pop, country "artist" requires little to no talent or musical knowledge .

Eddie Van Halen -- heavy metal, hard rock artist classically trained on acoustic guitar

Steve Vai -- jazz, jazz-rock, electronic rock multi-genre artist was classically trained on acoustic guitar.

Andy Summers -- jazz, rock, wave guitar artist former Police guitarist was classically trained by one of the greatest classical guitar maestro Emanuel Barrueco

Jan Akkerman -- classically trained on violin performs as lead guitar of jazz-rock band Focus

Jack Bruce -- classically trained on violin and performed as his most know act at Cream -- blues-rock power trio

Joey DiFrancesco -- jazz organist, jazz-rock -- self-trained since the age of 3 and performing since age of barely 7 at the stage (is there any talent?)

Irmin Schmidt -- electronic rock, kraut rock keyboardist and pianist -- alumni of Stockhousen Conservatory as pianist and orchestral conductor. Stockhousen Conservatory is school of innovative music and Jazz in Germany, but it also provides classical training. One of the most unusual schools of music in the world, but bare in mind that classical music rootes mainly from Germany and Austria and I believe still continue to add on...

There are many and many more rock or other ganre artists that literally infused J.S.Bach in their tunes and improvisations -- YES indeed he's one of the greatest, because his music still influences the most current tunes you hear -- just need to realize and just need to optimize judgments towards definitions of talent and musical knowledge. 

Country artists vary between family musical training and various schools of art. I may not find myself enjoying most of the country music, but I do find myself respectful to country artists skill to play instrument. When I hear Ricky Scaggs reefing acoustic guitar, I want to make it lowder, but when I hear his typical country style voice, I may want to shut it all the way down:-)

Classical music even nowdays still evolves, but not finding large enough listener due to commercial media. There are still classical composers of today finding their place mostly in the movie soundtracks. One of my favorite is Maurice Jarre.

I agree with @schubert , Bach was an one of a kind genius. His compositions are complex and beautiful. Anybody who plays the piano would tell you the same. 

czarivey. You seem totally unable to grasp  that in adult conversation some things are so obvious that the norm is to not  state them .
Of COURSE there are people in pop/rock, even country, that are talented and well educated as well.
However it is quite CLEAR that above is optional !

Not so many.

Banco del Mutuo Soccorso (- their 80´s stuff)  feat. Francesco di Giacomo RIP

Practically all RPI. Well, the 80´s wasn´t their finest hour but was it to anyone really in Prog. The early 80´s was rock´s decline anyway.

Ennio Morricone
Keith Emerson RIP

@schubert ,

it is quite CLEAR that above is optional !

What about Mozart who revealed his talent at the age of 7?
It looks like training was indeed optional for him and his music isn't so simple as well.
Later on he learned how to place his own music on our traditional western 5-line staff, but before that he was just like Art Tatum not knowing single note and he learned to sing prior he started talking perhaps in his mother's womb.

Today we have similar talents that play various types of music and sing, but not too much classical. The classical music performances are substantially more structured and strict today. The classical musician is expected to provide clean and precise performance rather than personal interpretation often used by self-taught and trained musicians. 
Once Joey Di Francesco  started learning classical piano somewhere in his early 30s, he quit not too far from his start, because it's not his bowl of soup, however he mastered to learn solfegio. 

One thing I can agree on is when there's no talent or training -- there's no music whether it's rnr or classical. The result is not interesting and boring.
As to commercial media, it's looking for talent that can bring profits to the industry so there must be some of both above mentioned to at least certain magnitude.