I think y’all misread the ending of Iron Man 3. Tony destroyed all his suits in the House Party Protocol as a gesture for Pepper, not because he was done being Iron Man. The suit he was wearing when he blew them up was the Mark 42. By Infinity War he was wearing the Mark 50. That means he built eight new suits between Iron Man 3 and Infinity War alone and the film never needed to explain it because building suits is literally what Tony does when he is anxious, grieving or bored. The more interesting detail is that between The Avengers and Iron Man 3 he went from Mark 7 to Mark 42 in roughly a year because of PTSD. So the precedent for rapid suit production during emotional distress was already established. Blowing them up at the end of Iron Man 3 was a promise to Pepper. Building new ones immediately after was Tony being Tony. What I find more telling is that by Endgame he was on the Mark 85. He built over forty suits after blowing everything up in Iron Man 3 and the MCU never felt the need to show us most of them because Tony building suits is as natural as him breathing. The destruction was emotional. The rebuilding was inevitable.