So let's look at the original statements made in the OP.
The .410 shotgun is a great defensive tool,the gauge/caliber can be used by anyone that can shoulder a long arm. Mostly true. It is more than adequate for defensive use. Most anyone who can shoulder a long arm could use one.
The ammo situation is being cleared up due to the handguns that have arrived on the market. True. Before the 410 derringer can onto the scene your choices were bird shot and 1/5 oz slugs. Now we have real choices in buckshot and better slugs are available.
The .410 guns are light weight and have a low recoil. Mostly true. The real lightweights like the single shots have a fair amount of kick with a 3" shell. Yes you can game a 12 gauge into the same levels of recoil with 20 gauge payloads and low velocity but that's not the point.
Ammo is also lacking in the weight and bulk area,which allows for more shells to be carried for the same weight as larger gauge guns. True.
The payload is small but is still capable of putting down a two legged threat and is nothing to sneeze at. True The lowly 2 1/2" shell with 3 000Buck pellets is still putting out 210 grains of lead at 1200-1300 fps.; that is 41 Mag. territory energy wise spread over three wound tracks.
For those that are new to shotgunning I would recommend a .410,not only because of the low recoil,but because the limited amount of shot will force the shooter to practice more and in turn become a better shotgunner with the heavy hitters on down the line. False. You can't make someone practice. The higher (almost 3X) cost of shells in general will turn many people away from practicing more. With the 12 and 20 gauge you can get bird shot loads that have the same weight and velocity to match the buckshot loads substantially cheaper than buckshot. For defensive practice it really doesn't matter if you substitute bird for buck; you either hit the target or you do not hit the target. The indoor ranges that do allow shotguns do not let you use buckshot anyways; at least in my neck of the woods.