Terry on moretprs wrote “What TPRS really makes is little, rather socially-isolated [=little vocabulary, as though they’d had little world experience] pseudo-native-speakers [people who instinctually use the structure of the language correctly and can easily acquire new vocab by attaching it to that structural framework].” This is what I see among a lot of kids who speak the target language at home: we call it kitchen Spanish or babushka’s Russian, but all the “hard parts” are there: the reflexive verbs, the subjunctive, the order of pronouns, etc. Some irregulars might be smoothed out, influence from English is common, but compared to a novice, they have a lot of the tough stuff. I remember the dishwashers at the hotel thought my Spanish was so great b/c I said “influencia” even as I totally butchered the idiomaticity of the language. Vocabulary is an add-on, grammar is the nuts-and-bolts or…. vocabulary is the bumpers and grammar is the internal combustion engine.