"My argument is that the bars, brakes and tyres, not to mention the geometry of your gravel bike are limiting factors and will inhibit you long before you are ever going fast enough to employ all 125mm of said dropper post."
I think you misunderstand what a dropper is really doing. A dropper allows you to access the bikes full potential by allowing you space to move above the frame. If you can move your body around more freely / aggressively by using a dropper then you can access more potential from your bike. A high saddle limits what you can do easily / safely on a mtb (if it didn't why would droppers exist). It's not a stretch to say the same for gravel so you perceived limits of the bike are 'high saddle' limits not 'low saddle / dropper' limits which will be further up the performance curve.
Useless they've mtb'd your 'average gravel rider' may have no idea what a gravel bike is capable of if only the seat was lower giving them confidence to find out, and potentially missing out on really fun terrain on their doorstep. It's NOT about speed - it's about control. Having more control just allows you to access higher speed safely.
I don't think it's weird to think that many average gravel riders would take in a (or many?) section of more technical terrain on their gravel ride. It totally depends on the terrain available and the objective of the rider, not the bike.
Personally, I think you'll see more droppers on gravel bikes in future (although they'll still be in a large minority ) .
Droppers don't have to be remote operated mtb versions. Much lighter, simpler, cheaper, occasional use 'lever under the saddle' versions are also available...
Going back to the quote though - my road bike has 160mm rotor and hydraulic brakes. My xc mtb has 160mm and hydraulic brakes. Even if mtb brakes are more powerful, of the listed possible points of control failure on a gravel bike, braking shouldn't be on the list. Modern hydraulic drop bar brakes are plenty powerful enough and have the modulation too.
I think what this article is trying to say is that Gravel bikes with a dropper post are a niche within a niche within a nice and for most terrain and styles or riding you'll get bigger benefits out of straight bars and front suspension way before the benefits of a dropper.
In short there is very little gravel riding where a gravel bike with a dropper makes more sense than a Hard Tail. But she doesnt actually say that for fear of annoying everyone whose bought a new shiny N+1 with a dropper.
These are all arguments that went through mountain biking as the dropper was introduced but now nearly everyone has one.
The difference in weight between a long and short dropper is so negligible that if you're going to have a dropper the advice should be always get the longest that fits and is in your budget.
In the UK a lot of the gravel riding doesn't necessarily need a dropper but the bikes designed with droppers are designed with different places and styles of riding in mind. Having said that, there's definitely regular places I ride that I could ride faster with a dropper. A simple example is stairsets, riding gravel bikes can easily handle stairs, except being bucked by an upright saddle. If I could could get my saddle out the way I could ride longer stairsets faster, and that would be more fun.
The bit about descending on the hoods is really bizarre and suggests the writer could work on their own skill a bit before writing polemics on how others ride.
For many, gravel bikes double as winter commuter bikes for the wet months, or everyday bikes to putter around with your kid or go to the local cafe. I've got single-sided spd pedals in mine, so the dropper post makes it easy to adjust seat height depending on whether I'm wearing flip flops or cycling shoes.
I am a little unsure about the point you are making in this article. In the title, and some places in the article, you are saying we don't need a long dropper post, but then in most of the article, you seem to be saying we don't need any dropper posts.