I don't think the is 16 obsolete. The 16 along with the 28 and 10 gauge would be reasonably considered niche gauges. Each has its adherents but with the introduction of magnum cartridges in the 12, 20 and to a lesser extent the .410 there are not quite the same advantages to the others as there once were.
You're right, to the extent that the 'noticeable' or 'practical' distinction applies. By that I mean that, while magnum shells will definitely approximate the same number of pellets in the shot string, and should actually improve shot penetration over a properly charged "square" load, they are over-driven and won't pattern as well. This is due to the 'flier' pellets that are stripped from the periphery of the shot group as it leaves the barrel. With a magnum shell, the shot string will also be longer (more strung out), producing a less effective pattern at the instant of impact on a moving target.
To the average shooter, are these 'practical' considerations? I doubt it. They wouldn't even know how to pattern the moment of impact on a moving target, or care to know. To someone not just willing, but eager, to spend thousands on a 16 gauge or 28 gauge Grulla or London best, is the difference noticeable? Certainly, it is. Very few owners of such guns would ever consider subjecting their guns (or their shoulders) to the punishment of a magnum load when they know they can get similar or better performance from a well-constructed shell.
I know that to the average shotgunner this is nothing more than snobbery and elitism. But, that's just a reflection of the value they place on the differences between the two approaches. When I first caught the shotgun bug, my primary consideration was getting one single shotgun that cost less than what I then considered a lot of money for a shotgun ($500) and could serve all purposes I had at the time.
As time passed and I got involved in clay sports and other forms of hunting, I didn't think it unreasonable to own more than one shotgun, each more suited than the others for a particular purpose (and each costing as much as my original total budget). Soon I began to realise that I enjoyed shotgun sports, both live game and clays, more than many of my previous pursuits. In fact, so much so that I began spending more on books than I originally spent on equipment. I began appreciating some shotguns for their unique or clever designs, or their historic value. Damned if I didn't start buying guns that I never even intended to shoot.
The virus within me has now progressed to the stage where I'm willing to let go of many of the guns I started out with. When I do, the money immediately gets reinvested, but now my focus is on acquiring state of the art pieces. To me, the state of the art was achieved a century ago. It has been maintained, and in minor ways improved, by today's most skilled craftsmen. But, no mass-produced guns even come close.
Magnum shells are a good improvement for mass market guns. They have not and they cannot replace or supplant an optimally charged and loaded shell in the gauges most suited to them. Until the London Gun Trials, this could all be dismissed as the ramblings of an elitist. The trials proved once and for all that, to the contrary, it is science that delivers proven results.
So, the distinction between 'practical' and 'noticeable' remains. I don't dispute anything you say, when taken from a practical point of view. By far, most shooters only care about the practical aspect. Those who appreciate 16 gauge and 28 gauge guns tend to have different motivations.
