I view it in terms of opportunity cost. Let's say your kid has 1 hour of productive attention span per week, time of focused learning. You can either teach them your native tongue or something else, like math and science, or better English.

Kids still pick up some vocabulary from speaking to their parents/grandparents. And even if they spoke it toddler-fluently before getting into preschooler, they won't be able to read a newspaper by the time they're teenagers. Without constant immersion, kids just don't have the discipline to keep going at it. This isn't necessarily a bad thing.