What would happen to US economy before US government lose control over its national debt?
It depends.  Mathematically, you just need GDP to grow at the same rate as the debt grows, and then you can sustain deficit spending indefinitely. But this simply shifts the line of questioning to "can GDP grow indefinitely", and for most people the obvious answer is "no".

Ironically, the band-aid solution that most developing nations have taken is to grow the population through immigration (ironically because this approach is seeing significant geopolitical pushback).  It's well-known that more affluent countries have lower birth rates, and so immigration is the "obvious" solution to this.  But it clearly carries its own challenges.