No one seriously proposes banning high-fertility individuals (Musk or otherwise). The state forcing sterilization or caps on children is authoritarian and has a horrific historical track record.
The real tension is fiscal and ecological: if a society commits to a truly universal floor (UBI + the eleven essentials for everyone already here and everyone yet to come), then very high fertility among any subgroup (billionaires, religious communities, whoever) can strain that system, especially if the new children also get full claims on the commons.
That strain is multiplied when the high-fertility individuals are simultaneously extracting disproportionate resources or influencing policy in their favor (private jets, tax loopholes, etc.). It starts to look less like โfreedomโ and more like subsidized dynasticism at everyone elseโs expense.