I think the distinction between primary and secondary language-learning is a distinction that reduces and obscures the realities of what is entailed in language-learning. Yes, it is true that if you already have language-proficiency in one language, you can draw on your knowledge to make sense of the language you are learning (i.e. translate) and it is also true that habits of thought generated in one language can interfere in the learning of conflicting modes of expression. However, who is to say that children learning language for the first time do not experience similar conflicts with cognitive habits they have developed pre-lingually?
The big difference between pre-lingual and post-lingual language learning, imo, is pre-lingual children are under constant pressure to relinquish the expectation of being catered to like a baby, which is a major encouragement to achieve language abilities that will help them solicit help that is refused when they just cry and throw tantrums. Adults cry and throw tantrums with regards to any kind of learning, not just language-learning - but they do so by insisting on relying on already-acquired proficiencies, such as a "mother-tongue" or English in which they are fluent.
Also, since internet has evolved broadcasting and point-to-point communication to an advanced level with global range, "full-immersion" is no longer a plausible scenario, imo. Or rather, "full-immersion" can only happen by a concerted decision to avoid relying on any assistance-language, such as English. However, there's no reason why people couldn't discipline themselves to avoid switch between languages if they could tolerate the struggle involved with searching for ways to express things and understand others with limited comprehension and active proficiency.
In short, language-learning like any kind of learning is a question of practice and patience, as well as other participants willing to practice with the same level of patience.