mixing hit songs (is easy)

Congratulation, You just "got it" 
 
Only record real good stuff and record it so it sounds as good as it's every going to sound. 
Don't hit that button until the part sounds right and can be played or sung at it's best. 
It will mix it self if all the levels were set properly too. 
 
I think a huge problem with unlimited tracking is people just record garbage and then try and make it sound good after the fact. It's why I don't even use Guitar sims. I get the guitar part to sound the way I want to fit the song,,, then I hit the button.. and I play my part all the way through if at all possible. 

Back then it was all about "Do it now" Vs. "Do it later".
Most of the time EQ, Compression and even Reverb/Delay FX were printed right to tape.
That was what I did and many, many others did as well. Matter of fact when I first started using Sonar I used this approach and used it for many years. It's a double edged sword as you can either capture lightening in a bottle or capture something that can never be changed. that is the sword of destructive recording.
 
There is something to be said about this way of working. you are left with data that is, in many ways un-editable.
Every mistake, every little nuance caught on tape in the moment. Joe Walsh has been a big promoter on this method of recording. His stories of how the album "Hotel California" was recorded really zooms in on this way of recording.
 
There are only so many "Do overs" you can get before it sounds too polished and over done.
Sometimes those little mistakes add up to become collective work of art. A good example would be to compare it to classic paintings done in, for example; 1600's thru 1800's. An artist used a brush to capture his visions.
I believe this is still the best way to work Vs. using Photoshop to fix little mistakes and add vibrance.
 
There are and will be many different opinions on the craft and how to work using both methods (early and present).
There is no debate though because everyone works in different ways and I could never argue anyone's opinion no matter how bizarre because they are artists and we should respect that. Face it, the best art has been born out of experiment, accident or breaking through the barriers of logic. Which one will you use ?
 
 
 
 

The digital realm has changed the way we work. As others have alluded to, working with tape, analog mixing desks and hardware outboard gear was a very different process that had many more limitations. Back then the time and effort used to go into capturing the perfect sound to tape, processing each sound as it was recorded to get as close as possible to the final sound. This was done for several reasons but the main one being the limited supply of hardware processors available even in a high-end studio. Working in the box with digital, the time and effort is spent in post recording - we tend to record unprocessed sound and then process it in the mix simply because we can as we have an unlimited supply of any one processor. In essence, the amount of effort is the same but happens at a different stage, so while mixing from tape seems easy, it's because most of the processing has already done in getting the sound to the tape in the first place - premixing. The final mix will just involve balancing and tweaking and the processors you used during recording are now freed up again for finalizing the mix.
 
The above is the basic answer, but there are a few other factors too. It's easy to over complicate stuff in a DAW. The old adage "less is more" is worth remembering. There are also many more things we can do in a DAW to improve the mix beyond what we could have ever done before.

There's no doubt that great performances are vital, but it's also worth mentioning that many of the publicly available multitrack masters are actually stems.

For example, the original recording of Queen's 'Bohemian Rhapsody' includes 180 tracks, the majority of which were used for the vocals. To accommodate this, several 24 track machines were chained together and later mixed down to a 'more manageable' set of stems for mixing purposes. I know this is an extreme example, but many of the available 'multitrack masters' have been through a similar process.

Brian May goes into detail in the following video...
 
Mixing Bohemian Rhapsody