Did you take physics in high school or college?

Your question is similar to..... "What is the difference between speed and acceleration?"

Just because you are "continuously acquiring quality back-links" doesn't mean that your competitors are sitting on their butts.

If you are gaining ten links per day but the guy above you is gaining twenty you will never ever catch him....

.... and if he is on the third page then the guys near the top of page two might be gaining hundreds per day.

Every time you move up a position in the SERPs the website directly above you will require a greater level of effort to defeat.

That means you gotta press the accelerator down harder and harder as you move up the SERPs.  Will you have it floored before you reach the top of page two?

In lightly competitive SERPs you might be able to defeat everyone... but when you get into the heavyweight SERPs the increasing competition will at some point be more than most people can muster.

That's where you hit "SERP lock" as you call it.

Keep in mind that the people behind you are working hard too..... you might  tramp the pedal to the floor and see people from behind passing you buy.

My personal opinion is... more people who hold any SERPs today will be lower in the rankings than will be more highly ranked by this time next year.  Really.  They are going to be displaced by existing heavyweights who are expanding their reach and new heavyweights who are starting to accelerate.

Pick ten SERPs in different niches that interest you and record who is in position #5.  Then come back in a year and look for them.  More will go down than up.

I believe that google is smart enough to tear the wrapper off of a website in a way that leaves naked content.

While a lot of people do what you are proposing and may gain some benefit from it, I think that the benefit is very small.

It's another case (in my opinion) of allowing the minute details of SEO to have an undo level of control over how you run your website.

At my office we do not have any SEOs or designers or content writers or developers.

Everyone here is a "webmaster".  A job that requires broad expertise and responsibility.

Working in silos is ineffective.

Most of the 'low hanging fruit' that I have picked has been figuring out ways to make more money from my current traffic rather than going out after new traffic.  If you are working on an established website with good traffic it will probably be easier to double your income from current traffic than it is to double your traffic.  Better ad placements, more effective paths to YOUR goals, more enticing descriptions, more obvious calls to action are examples.

Get Tim Ash's book... Landing Page Optimization.

Other 'low hanging fruit' has been simply knowing my products and discovering SERPs where I have no presence or an unoptimized presence and building an attack on them.