it would not be worth the investment given that humans have a very high chance of either dying when they take wounds requiring a dread or dying after being interred in one
I would guess that the DAoT humans who designed them originally probably had a different interface mechanism, which was probably lost.
Curious how quickly do people think the Ultramarines with us can rebuild when given the resources?
It depends on how many extracted progenoids they have in storage. At minimum, they should have 15 (from their current recruits), but it's quite possible they have more due to any extracted from those Astartes that died. So the speed will depend on that.
A Space Marine's implants have to be put in between the ages of 10 and 18, with 14 being the latest for the early implants and the last by age 18, or 16 at the youngest. Ideally, anyways. So it would take 4 to 8 years to do the implantation alone. Before that the Aspirants have to go through a selection and training process to pick the best candidates, which will take a few years. Once implantation is done, it takes 5 more years for the first progenoid gland to mature for harvest.
So I'd guess you're looking at something like 15-18 years to get through the first cycle of recruitment, but you can shave the selection/training time off the second onward since that can be done concurrently.
So if they have only 15 harvested progenoids available, the first batch will give 15 Astartes, and then the next 15, and so on and so forth. So linear growth so long as no more Astartes die so the second set can be harvested. It would take them 65 cycles to reach full Chapter strength at that growth level, so nearly a millennium. Of course, they could accelerate things by harvesting the second progenoid early, but that goes against tradition. But if they've got more than 15, it will go proportionally faster.
Well, that's if one progenoid gland = one Astartes, anyways. If you can get at least 2-3 Astartes from one gland it'd go faster.
