You respond too fast. I was busy writing my response.
Dracula's soul would have had to hitch to SOMETHING connected to Soma prior to his actual birth before he could inherit it. I'm saying that he wasn't truly Dracula's Heir until his physical birth. During the 9 months of fetal development, he was just another "bun in the oven" and stood no greater a chance at being Drac's heir than anyone else on the planet. By coincidence or fate (though I'm heavily on the fate side) Dracula's "shattered" soul, his
spiritual remains, latched onto Soma, making him the One True Heir.
This carries continuity with the series, as Dracula needs a set of remains to return to, be it the power he gave the Devil Forgemasters like Hector or Isaac (actually a pretty cunning contingency plan imo), physical remains like in Harmony of Dissonance, Simon's Quest, or Symphony of the Night etc. al, or in this case, his spiritual remains.
After 1999 everything that was still left of him was metaphysical: there were no more Devil Forgemasters (so he couldn't hijack a body), his physical body was either obliterated completely (most likely scenario), or trapped in the Eclipse with the Castle making it useless for the purposes of a resurrection (less likely but still plausible).
Without physical remains to return to, it latched to a human boy
as he was born. Whether this was on the day Dracula died or not is ultimately insignificant. The birth alone was the opportunity Dracula's essence needed, and I will explain why.
Warning: shit's about to get real heavy and complex.
In real world magical theory, birth is a key threshold -- lending additional credence to that idea that prior to birth Soma was just a human fetus, but after birth he was Dracula's heir and reincarnation. Iga probably didn't realize that when he and his guys penned it; they likely just assumed that "yeah, that sorta makes sense and sounds pretty neat so let's run with that". But it matches up, regardless of why it was written that way.
Thresholds like a New Moon, Full Moon, Birth, Death, or indeed a TOTAL SOLAR ECLIPSE are all massive in scope, power, and vulnerability.
Concerning the 1999 Event and the times that lead to Aria's 2035 setting, we get several doozies of thresholds that would set up for Dracula's reincarnation as Soma.
Dracula died in 1999 (first Threshold) during a Total Solar Eclipse (second Threshold)
that in real life occurred on August 2nd, 1999 and was in fact visible from Europe. Dracula's essence fuses to Soma at the moment of Soma's birth (birth being the third Threshold). Now, Soma is 18 in Aria of Sorrow, which places his birth in 2017.
There will be a Total Solar Eclipse that passes over the United States in 2017 on August 21st. This is the fourth Threshold, and it is presumably when Soma is born and Dracula's essence latches onto him. Furthermore,
there will in fact be yet another Total Solar Eclipse on September 2, 2035 AD in the real world... and it passes right over Japan. This is doubtless the fifth and final Threshold that opens the way to the Castle for Soma and those caught up his destiny.
In addition to now knowing the most likely dates of Dracula's final death, Soma's birth, and the events of Aria of Sorrow, we now have a chain of serious magical thresholds of power and vulnerability that make a clear path for the reincarnation of Dracula, bolstered by actual real world data that would line up PERFECTLY with what we know of Soma's life.
Really, Dracula's reincarnation was an astrological certainty.
But none of this detracts from my initial theory that Dracula by 1999 was distinct from Mathias, and that Soma's life simply finishes the redemption that Alucard began for his father in 1797.