The evolutionary process in bird wings, especially with regard to winglets
I looked up winglets so I had context for this answer. I'm interpreting winglets as the vertical tips at the end of airplane wings. If so, then you are correct. The spread primary feathers of soaring birds like eagles function as winglets (Tucker 1993). Airbus has a biomimicry web page devoted to some of the biological designs, including winglets, they incorporated into their airplane designs. Some studies suggest airplant winglets do increase efficiency (e.g., Hossain et al. 2011), but there is still some debate.
From the aviation.se answer:

Look at birds: They use two different wingtip designs. I guess we will both agree that those designs are mature after millions of years of evolution. And still there are two distinct wing tip shapes: One is the &quot;fingered&quot; wingtip with large, spread-out feathers, and one is the pointed tip you find from seagulls to albatrosses. Why is there not one, mature design?
This has to do with the environment they are used in. Seabirds fly in an unobstructed environment with steady winds and need to stay aloft for longer duration. All other birds have to cope with trees obstructing a straight flight path and gusts from hills, or those trees. They need to maneuver quickly and cope with gusty winds. This is helped by a reduced span and the possibility to fold the outer wing in or fan it out in an instance. Hence the fingered wingtip.

Although bird wings do tend to match his descriptions, he is not correct that these correspond entirely to ocean-going birds vs all other birds. Many inland birds have pointed wings, including swallows, nightjars, swifts, and falcons.  Some seagoing birds have winglets, such as pelicans and cormorants.  While wing type is shaped by natural selection in a given environment, it also reflects the evolutionary history of the birds.
There is not &quot;one, mature design&quot; because wings many different shapes and sizeshave evolved under varying selective pressures. For example, the long pointed wings of ocean-going albatrosses are excellent for gliding with little energy expenditure but they are lousy for taking flight or for landing. Other wings are shorter and broader, providing greater maneuverability in the woods. In this sense, there is no perfect wing. Each has evolved for different evolutionary reasons.
The main question
From the aviation.SE thread:

the fact that no random mutation has produced a winglet* in birds is in no way a demonstration that airplanes do not benefit from them, there is no logical relationship. [*even though the &quot;fanned&quot; design you refer, such as the eagle's, can be considered a form of optimum winglet] –  Federico

followed by this response:

So you consider evolved wings as not mature. The winglet modification is just waiting to happen? Nature never tried it, in &gt;100 million years of biological flight? Could be, yes. But is extremely unlikely. That settles it for me...

Federico is correct. The basic wing type was set long ago as birds evolved from non-avian dinosaurs. Comparisons of modern bird wings with fossils suggest that the basic structure of bird wings has not changed very much since then. The current structure may be the best that has evolved by natural selection so far but that does not mean the wing cannot be improved further. As you noted, mutations could occur in existing genes that lead to further improvements of the structure and function of the wings. If those mutations never occur, then the improvements won't occur. This is one constraint of the evolutionary process (Hoffman 2014). Natural selection can only work with existing traits, using existing genes and genetic variation.
Another consideration is that wings for powered flight have evolved independently at least four times (birds, bats, insects, pterosaurs). Each time, a different type of wing evolved. Is one better than another? Bat wings have aerodynamic properties very different from birds but function very well for bats (Hedenstrom et al. 2009). Along the lines of your argument above, each wing type perhaps is (or was for pterosaurs) optimum given the environmental circumstances and evolutionary constraints. That doesn't mean they could not be improved.  If a wing evolves again in some organismal group in the distant evolutionary future, odds are very good that the structure will be different from those that have evolved so far. Perhaps that wing will be even more efficient than current wings.
Spandrels

Another user cited the &quot;Spandrels&quot; in comparison to the above debate, could someone explain what could have been the meaning of the comparison?

The use of &quot;spandrels&quot; in evolutionary biology stems back to a well-known paper by Stephen Jay Gould and Richard C. Lewontin called &quot;The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptionist programme&quot;, published in 1979 (Gould and Lewontin 1979). A spandrel is the space that forms between two adjacent arches or between an arch and a rectangular shape.  The spandrel emerges as a result of putting two arches next to each other and filling the space. It results from the joining of two arches. It was not specifically designed as part of the arch. Gould and Lewontin claim that evolutionary biologists are too quick to assign adaptive value to every trait on an organism, and argue that not every trait results directly from adaptation. Instead, they say, the trait may be a &quot;spandrel&quot; that emerged as a by-product of other traits. The &quot;spandrel trait&quot; has no adaptive value in and of itself.
The use of spandrels in the comments of the aviation discussion is not clear to me. Clearly, the person intended the evolutionary meaning, mentioning the paper by name. But I cannot tell at all the person's intent.
Edits: Incorporated comments by @Federico and @Andrew - thanks. I corrected horrendous typos and attempted to add clarity.
Citations
Hedenstrom, A. et al. 2009. Bird or bat: comparing airframe design and flight performance. Bioinspiration &amp; Biomimetics 4 015001. doi:10.1088/1748-3182/4/1/015001
Hoffmann, A.A. 2014. Evolutionary limits and constraints. Pp. 247-252. In: The Princeton Guide to Evolution, Princeton University Press, USA.
Hossain et al. 2011. Drag analysis of an aircraft wing model with and without bird feather like winglet. International Journal of Mechanical, Aerospace, Industrial and Mechatronics Engineering 5: 30-35.
Tucker, V.A. 1993. Gliding birds: Reducion of induced drag by wing tip slots between the primary feathers. Journal of Experimental Biology 180: 285-310.