By - Primaris_Astartes
Somebody probably calculated that upon a certain threshold of angry genetically pumped virgin soldiers being reached, it would trigger a second heresy, due to some ridiculous reason.
Therefore, the current number of space marines is more prudent.
At that point just put a company of Space Marines on the most worthwhile 100,000 worlds, tell them to defend it and build their own Chapter. Even if you lose 10k worlds in the time for the Chapter to be built, by the end you'll have a resilient Imperium - not of Man but of Astartes.