It has been an enjoyable read for the most part. I'm just a newbie here from the Netherlands (Europe). I enjoy GPS sailing since I started again in 2013 and bought a bunch of older falcons and some new free-race Gaastra-Phantoms. Some might say, why do GPS-(speed)-surfing on slalom gear, but hey that's what we do right? Yes I still love my good old Falcon speed, capable of doing much better speeds than my skills reach. That much of the introduction.
15 years is almost the difference between my Fanatic Falcons 2007 and my current iSonic 2019. Man, those falcons were really quick, at least the ones with flat bottom (speed (66), 80 and 105). I also had a 124 that had concave and was comfortable but not fast. So flat and narrow are fast, regardless the age of the board.
Mast-track position and strap positions, thickness of the board all moved backwards over time and also need different sails/pressure (low/forward) points. I did own a series of 2015 iSonics (w54, 87, 107, 120) and I think they are faster than the 2019 series (50, 63, 72, 83) that are more comfy. Wider at the back with cut-outs to control the width. So the average speed and gybing becomes a bit better compared to the older ones.
I never owned sails from 15 years ago. I jumped from the Gaastra F1 total-flow ('97) to Gaastra Phantoms (2012). I never had any big sails in the 90's just a Mistral Stinger with a course-slalom pro 4.7 and 6.2. With all the gear that I have now, I have a lot more sailing days. Did the sails get faster? I doubt, but again more controllable with stable pressure point. So what changed the most? I would say the (carbon) fins, especially the larger ones with specific flex/lift/control for the bigger boards. Again make it a lot more controllable. So we go faster with less wind with more controllable gear, but it hasn't improved greatly our top speed results right?
So what makes us go faster? More wind, better angle, flatter water and guts. Yes, less drag as well, but how much drag can we reduce compared to those ultra narrow needles with short asy fins and our aerodynamic sails nowadays? For the most of us, it is skill as well, not just on sailing but also on trimming. Do we take enough time to learn our gear? Even Vincent keeps his old sail and board because he knows it well. I can say that I need several years to learn and trim my gear. Not enough sessions, too much variations. So I change gear after 5 years.
GPS is pushing myself at any time. Getting direct feedback on my session, if my current run is faster than the top 5 that I already did is pushing me and my material. Get's me energized and pushing me to my limit. Is GPS another dimension to the sport? Definitely yes. It's an individual sport that all of a sudden gives a method to compare me against others, locals friends, PRs. It should not become an obsession to pursue and forget all about the fun of the sport. I admire the ones hitting the top ranking consistently.
Do the absolute numbers go down over the years? Really don't know. The biggest limiter for me is myself and the time I'm able to spend on this great sport. Even though I just turned 50 years old/young, I don't have the strength now that I had 15 years ago. Perhaps when I put more effort in that, I might improve a lot more than buying newer equipment.
Since there are too many variables, we should ask if there could be a Luderitz Classic challenge. Not on the top/record day, but on a consistent one before or after. Same rider only changing 1 thing at the time. Let's ask Ben Proffitt to do an idiots guide to speed-surfing part 2 with various (classic) gear. We might be surprised about the outcome. Sails/masts might be the trickiest part since they do wear the most and might introduce (safety) risks. I would go flat-out on an old board or fin, but do you trust the old sail/mast? Only then we'll know the answer to this topic.
So here my stats over the years from
www.gps-speedsurfing.com/mygps.aspx?mnu=GraphMySpeedStats 
Not so much differents in trend 2s, 10s, 500m, avg. Only the nautical mile is different since at my home spot it is normally too narrow to pursue this score...

Compared to the average peer I continue to improve. Not sure why 2021 and 2022 are missing...
Stil did not hit the 40knots mark as you can see, almost there...
www.gps-speedsurfing.com/?mnu=user&val=342902&uid=7469