It's simple, really.
Anything with a fan inevitably will degrade in performance over the years due to its mechanical parts (the fans, mainly) degrading.
A computer like the MacBook with very minimal heat output and essentially just a heatsink potentially will not degrade in performance the same way as a machine with mechanical components does. If you have not noticed already, we were sort of moving away from too many moving parts over the years. First, the optical drive went away, then spinning hard drives were replaced with solid-state drives. Now we have fanless computers that put up reasonable performance.
People who are new to the Mac family probably do not know, but for those of us who have been with Apple a long time, we know the machines with fans will eventually degrade in performance very significantly. The fans just can't perform at 100% capacity forever. That's not physically possible. Either they have to be replaced at some point (along with the heatsink and the thermal paste), or you just need to upgrade altogether.
So personally, I have waited for a fanless MacBook forever, and I finally got my wish in 2015. People did not realize the significance of the machine because they are too used to other machines with supposedly higher specifications, but... allow me to remind all of you, that in 2009, Apple thought a dual-core processor with less horsepower, less cache, significantly less RAM, than the current MacBook with no fan, was the top of the line (the 15" MacBook Pro).
Compare:
MacBook Pro 15" mid-2009
CPU: Intel Core 2 Duo 2.66GHz w/ 3MB L3 cache
RAM: 4GB DDR3 1066MHz
Storage: 320GB HDD
MacBook 12" 2016 base model (the m5 and m7 models are much faster, still)
CPU: Intel Core m3 1.1GHz (can be boosted to 2.2GHz) w/ 4MB L3 cache
RAM: 8GB DDR3L 1866MHz
Storage: 256GB SSD
That 15" computer in 2009 could do all of the things people are doing now: photo editing, video editing, web browsing (with much more resource-intensive Flash, by the way), 3D modeling, and office works (documents, spreadsheets, presentations)
Aside from display resolution, and by the way, the 15" computer from 2009 also supports a 2560 x 1600 external display that is obviously higher resolution than the MacBook 12" internal, what, pray tell, has actually changed that warrants anyone needing more processing power to get the same tasks done? If you're going to say "operating system", then I'd say Apple has really gone downhill a lot since 2009 because their OS essentially does the same things as back then, with some extra bells and whistles (unnecessary) that cause performance degradation overall. Seriously, please think about that.
I think we are just too spoiled by the progress (or in the case of software, I'd dare say it's more like "regression") of technology, honestly.
Yes, the MacBook Pro are faster, and can achieve certain tasks faster, but for most other uses, the MacBook 12" should not fall far behind.
With that said, my 2012 rMBP 15" has had almost 5 long years of work, and the 12" has retired it. In actual use, I don't notice much of a performance difference between the two, if at all. And my use also includes regular "heavy" Photoshop and Lightroom work.
Again, I think people are just too spoiled these days and they require everything to respond lightning fast... within 1 second or it's a "defective" product. Personally, I lived through the good ol' days when we had to wait an hour for a floppy disk (1.44MB!) to copy its data over, and whatever we have now is so very luxurious already. Until we get virtual reality, that is.
