First of all, there is no "standard" Muay Thai stance, other than when westerners are teaching Muay Thai, and theorizing about it. There are all kinds of Muay Thai stances, everything from Samart's extremely narrow stance that looked like he was standing waiting for a bus (it had nothing to do with checking kicks, and he had no problem winning a WBC world boxing title), to wide stanced, front foot fighters all throughout its history. The problem is that so much of Thailand's Muay Thai is filtered through very select examples, and through western commentary on "why". If you want to see how a narrow stance can track down mobile fighters watch the footwork of The Elbow Hunter of 100 Stitches, Yodkhunpon Sittripum. The guy could close distance like nobody's business, but held a pretty narrow stance.