Here's a good, short article on Colin Montgomerie and David Duval advising Jordan Spieth "chase improvement at your own peril": http://www.golfdigest.com/blogs/the-loop/2015/08/montgomerie-duval-warning-to-j.html
As a former highly competitive golfer that saw his ballstriking fall off the planet when he got into college and had to work diligently thru the years to get into being a serviceable ballstriker, I can relate to much of this. And I agree with Duval and Monty on the key point, you have to be careful about changing anything in the swing in hopes of making improvement because it may be due to your detriment. For any full-time Tour player, the fact is that you're more likely to regress than you are to improve if you change the mechanics of your swing.
For every Jason Dufner who greatly improved his ballstriking once he changed his swing, there are probably 5 players that changed their swing and essentially got worse. Either by seeing no difference in their ballstriking, but spending more time on the swing so their putting and short game suffers or the golfers that just get worse with their ballstriking. Believe me, I see the statistics all of the time and you would be surprised of the number of players that make swing changes and see no difference in their ballstriking or get worse.
My feeling is that what most golfers that are good enough to play on Tour should focus on is sustaining their their swing over time. I think their inability to sustain their swing over time is where most players tend to get into trouble. And I think that was what caused the downward spiral in my own swing when I playing college golf.
For whatever reason, I started to incorporate new mechanics in my golf swing over time. I didn't try to do anything, but my swing started to change and I didn't even know it until a friend pointed it out to me. And even then, I still didn't believe my friend until he showed a video of my swing which looked completely different from past videos of my swing that I would film about once a year.
I was essentially self-taught up until that point. By being self taught I had almost zero swing knowledge. My swing knowledge was so poor that I used to think you wanted to take a divot directly underneath the ball instead of out in front of where the ball was located. And that lack of swing knowledge put me in a precarious position because if my swing were to fall apart (which it did), I:
a) Didn't have any idea on how to fix it.
b) Had no idea of what instructor to see that was adept enough to fix my issues.
I think what happens with players that do not sustain their swings is that their performance finally drops off to a point where they can't accept it and then they try to figure it out, but they have no idea what they were doing in the first place. And then they may go to an instructor and the instructor has no idea what their swing looked like to begin with and cannot reasonably get themselves swinging like they used to. Perhaps they are able to get back into some of their old swing mechanics, but they are missing a piece or two of their old mechanics and it just doesn't work.
That's where I find quality instruction very important. Have an instructor that knows your swing and has video of your swing (or even motion capture and Trackman numbers which are not mandatory, but can be helpful). Then if your swing starts to naturally change and create lesser performance, try and figure out what the old swing was doing and what has been altered.
I think most people will pinpoint this change in philosophy of changing your swing despite being very successful with it on Tiger when he changed instructors and swing mechanics from Butch Harmon to Hank Haney.
However, I think the player that started it was actually Nick Faldo. Here's a video from instructor, Lucas Wald, on the swing changes Faldo made and his success on Tour before those swing changes were made:
Faldo's swing changes along with his training philosophy changed the world of golf instruction. Before, there were not many full-time golf instructors. Most golf instructors actually worked managing a golf course or driving range and then they would schedule lessons to provide a little extra income. When I think of full-time instructors during that era, the only names that come to my mind are Bob Toski, Phil Ritson, Jack Lumpkin and Jim Flick who used to have these Golf Digest golf academies.
Faldo's work with Leadbetter created a paradigm shift into full-time instructors all over the place and what we see today...instructors that almost exclusively just teach players on Tour and travel to every single event to work with their clients.
While Faldo may have started the entire philosophical shift, Tiger really popularized it. And once he kept winning with Haney, then the belief that you could just simply change instructors and insert new mechanics without issue started to take hold.
However, if you look at the metrics that I wrote about in GolfWRX, it was not cut-n-dry as far as Tiger's changes to Haney went with his ballstriking:
Under Haney, Tiger's driving regressed considerably. However, his iron play became so impeccable that it may have become the greatest stretch of iron play of all time. Furthermore, Tiger's putting really took off. When Tiger was with Butch, his putting wasn't nearly as good. He appeared to have a couple of incredible putting years under Butch, but he also had some struggles as well. The thing is...when you hit it as well as Tiger did and as long as Tiger did under Butch...you can still dominate and not putt all that well.
As I pointed out earlier, a lot of times players change swings and their ballstriking is no better or no worse. My judgment from the numbers is that Tiger's ballstriking probably was a hair worse under Haney, even when he was winning left and right, than it was under Butch. So I think for all of those 'Tiger changed from Butch to Haney and had success' advocates, they really need to look at the big picture and ask 'did he really improve his ballstriking with the swing change?' And by the end of Tiger's time with Haney, his swing looked nothing like it did when he first started working with Haney.
So yeah...chase improvement at your own peril. But, you're better off focusing on sustaining what made you great in the first place.