List of artists that have never lost quality throughout entire career

I have the same feeling for Steve Howe.
He's probably got arthritis affecting his wrists and he can't play clean anymore as he used to play
He doesn't trip, but has trouble keeping sound clean against frets. It happens with aging musicians. Andres Segovia closer to the end of his career also sounded bad.

In Bruce Springstin I saw and heard everything: arrangements, good release management, sound, mastering, good live concert management, but, unfortunately, no music! It seems always to me that music isn't Bruce's cup of tea at all. Song "Born In The USA" heard on car radio triggers my wrist to change station few seconds after it's heard.

With new technology today and new training programs available for musicians, I can say that there are plenty of organists that could possibly outplay J.S.Bach even playing his own pieces. Since we can't hear J.S.Bach recorded, we can't really judge...
Remember, history is never precise

Tom Petty than is fraction of Willie Nelson who’s career is way over 40 years.

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.

@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.

It's true with many classical performance ways to define moods and change pace towards benefit of a certain instrument or a sound, but that's not defined as improvisation.

@dragunski 
Most of Brandenburg Concerti played mediocre or boring perhaps except Richter piano transcription.

I noted Gentle Giant here.
They were good first 2...3 years and than went to pop.

Larry Carlton has his career downtimes. Lately he is very boring