Basically.... job stones are a fucking pain in the ass. The jobs changed so much since the beginning and, as someone already said, they will continue to change as much as it's needed through the next expansions.
Aside from the blackmage where it's crystal clear it's impossible without a job stone, the rest of the jobs, or more precisely specialized disciplines, job stones are not really necessary. They are just here to make someone learn the discipline faster, or to pass on the techniques to the next generation, but in no way jobs are bounded by their job stones nor a job is a job just because it has a job stone. If we go like this, we could simply take a soul crystal and infuse the techniques of a gladiator into that stone.
Concerning the dragoon, if I'm not mistaken, the drg soul crystals take their share of power from Nidhogg's eyes, only the Azure Dragon is given the eye to benefit from its full power. Moreover, even if a newbie gets his hands on a drg soul stone, it does not mean he has what it takes for the crystal to awake and let him learn the techniques inside it. But it absolutely does not mean either he can't learn the job without the stone, with training and perseverance all is possible, and either not going to the length of Shattotto spending one's life on the same thing for black magic.
Again, Samourai's soul stone is completely useless aside learning the job faster. There are martial arts teachers everywhere in the East (you see them a lot in Bozja, and have some details in the Field Records).
Job stones seem to bind the job to them only from an Eorzean view point, it's much less of a handicap in the East and in the First. From a RP viewpoint, it does not prevent you from learning a similar discipline and having the same power as a "job with a job stone".
But as I said in the beginning, soul stones are a pain in the ass to deal with, so it might be better to ignore them or just stick to the "learning faster or pass on to the next generation" for most of the actual specialized disciplines whether it be for lore or RP purpose.