Pyrus said:
Regarding the lack of successors, I know for the earlier foundings that it would make sense to look at the near-annihilation of the chapter at the Massacre. For a very long time the Salamanders were sidelined while they recovered their numbers. I would go so far as to say that this massive rebuild depleted the "reserve" geneseed that the successor chapters were drawn from. It may be that the Salamanders have few, if any, successors due to the simple lack of surplus geneseed. Not to say that the chapter is at the edge of collapse, but I'd imagine that there would need to be a certain level of overstock before spawning a new chapter would even be considered. A wild guess on my part... they may even have a bit of an institutional paranoia about having enough geneseed hidden away to rebuild if something like the Massacre ever happened again. This could easily translate into an unwillingness to part with any more than the Mechanicus tithing requires.
And that would have a knock-on effect. There are supplies of Ultra Helix geneseed coming in from over five hundred Chapters during any given tithing cycle... which means that there's a large stock of that geneseed. Having produced few successors, the Salamanders geneseed thus exists in relatively few places and thus only makes up a tiny proportion of the geneseed tithe... as time goes on, the relative proportion of Salamanders geneseed to Ultramarines geneseed (to continue the example) doesn't change, so there's never going to be very much Salamanders geneseed available.
I mean, yeah, there's Horus all mortally wounded, and who pops in but Dorn the goon and the custodial crew...sheesh! what's a demon-possessed madman with dreams of godhood to do? F@%k!!! And now he's a slave of the Imperium he sought to rip down! Geah!!)