Why can’t more older 737s be retrofitted with more newer winglets? Some background: there are four different types of winglet that have been used on various 737s over the years.


The mini winglet is used to retrofit 737-200s (one of the two 737 Original variants, the other being the 737-100).  It is basically a tiny trapezoidal tab riveted to the rear portion of the wingtip.




(Image by Julian Whitelaw at airliners.net, via meisam nemati here at AvSE.)


The blended winglet is what most people think of when they hear the word “winglet” - an attachment that looks like someone simply extended the wingtip and then bent it up to near-vertical.  It was factory-installed on all 737 Next Generation variants (the 737-600/-700/-800/-900) for the first two decades of production, and has also been retrofitted on most of the 737 Classic models (the 737-300/-400/-500) that remain in service.




(Image by Ralf Roletschek at Fahrradtechnik auf fahrradmonteur.de via Wikimedia Commons, modified by Altair78 at Wikimedia Commons.)


The split-scimitar winglet has replaced the blended winglet on the last few years of the Next Generation production line; it is shaped somewhat like the blended winglet, but a) is swooshed rearwards at its upper tip, as opposed to the simple trapezoidal tip of the blended winglet, and b) has an additional scimitar-shaped, rearward-swept fin projecting from the outside of the upbend in the winglet.




(Image by Mnts at Wikimedia Commons.)


The MAX winglet is used on the 737 MAX series (737 MAX 7/8/9/10); it, too, is a split winglet,1 but of a simpler shape than the split-scimitar winglet, looking much like someone extended the wingtip outwards, then split its upper and lower surfaces away from each other and bent them upwards and downwards, respectively, to form what looks remarkably like a glorified Airbus wingtip fence.




(Image by Aka The Beav at Flickr, via Helmy oved at Wikimedia Commons, modified by Altair78 at Wikimedia Commons.)

Now for what I’m curious about:


Although the 737-100 (the original Original) was produced in smaller numbers than the 737-200 and is no longer in service, it still hung on with various operators past the turn of the millennium; why weren’t any -100s also retrofitted with mini winglets, given that the -200 is basically just a stretched -100 with more fuel capacity?
Given that the blended winglet, despite being originally designed for the Next Generation wing, was retrofitted on the vast majority of the Classic fleet with little to no trouble, why couldn’t it also be retrofitted on the remaining Originals, given that the Classics are essentially CFM56-engined, (sometimes) stretched -200s?
Seeing as the Next-Generation-designed blended winglet was easily adopted for the earlier Classics, what keeps the split-scimitar winglet, likewise designed for the Next Generation wing, from also being retrofitted to the Classics?


(Yes, I know that this is three questions rather than one, but I felt it was better to have one question asking three closely-related questions than to clutter up the homepage with three separate but near-identical questions.)



1: As for why Boeing went to the trouble of developing two different styles of split winglet for the two in-production 737 families, that’s a different question.
Winglets change the spanwise loading on the wing. By increasing the effective span of the wing, lift is increased towards the tips of the wing. With more lift there, you get an increased bending moment on the wing.

All this means is that you can't just put winglets on or change the ones you have. You must retrofit the structure of the wing which is very expensive and requires significant downtime. It also adds a weight penalty which decreases the benefits of the winglet.

For example, on the A320, which was already wingtip fence equipped, going to the sharklets requires removing the skin and reinforcing both the tip and the center wing box near the fuselage. This takes the aircraft out of service for 15 days and involves, I've heard, in excess of $1 Million in parts alone.

On the 737, the list price for the blended winglets is $1.06 Million.

So the answer for why is because it often is not economical to do such a huge modification given the limited lifetimes on older models.