Not saying it won't happen,but aren't you overestimating the efficiency and the abilities of the Spanish fleet?This is just before the Age of piracy.Given how much gold the conquistadores would have,it might actually kick start a whole smuggling business.As for substituting French or English settlers,is it really such a problem?You will also be getting settlers from other places like Italy and Germany as well,so you don't have a single large ethnic group,but others to balance them as well.Another thing is that given there's gold in the Americas,what's stopping a lot of Spaniards from migrating through the aforementioned smuggling business?Seville trade in the 1500s was for basic needs such as textiles, agricultural produce and ships... the English, Dutch and French were really only active in the region starting in the late 16th Century and in any case, trade with them would have been interdicted by Spanish fleets. America could/would become self-sufficient in time but that itself would require settlers and slaves to set up industries and to replace native depopulation.
Settlers rooted colonial administration over the Native Americans. The ecomienda system had each settler be the 'guardian' of a set number of Native Americans, the latter owing the former labor services. The arrangement restricted Native autonomy, an incredibly important consideration when the Spanish were still very much newcomers and minority rulers in the New World. Substituting for French/English settlers, even if possible, simply means giving up Spanish rule for their rule.
The legitimacy of the capitulacion was hugely important for the conquistadores, as through it they received royal sanction/protection for their work. A conquistador who broke his capitulacion and even worse, rebelled against the Crown probably opened himself up to assassination from his fellow conquistadores, who stood to 'inherit' the titles/profits originally assigned to said conquistador.
I think the main problem would be getting the trade started(ie finding a ship to get to Europe and then jockey with foreign courts to get their support in the endeavor).Once you manage to get the trade started,there's very few things Spain can do.Not saying all of what I'm proposing is easy and the chance of failure is quite high just like how the crusades and the conquest of the Americas depended on a streak of good luck as well.
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