I guess I can see that, but isn't it unlikely that the monk will have that much ki to blow? Flurry of blwos costs a point of ki, and then wouldn't each stunning blow attempt cost an additional point? Considering the fact that the monk likely spent a good chunk of his ki leading up to the solo boss...
Sure, that is the extreme case. A basic monk who has maxed Dex by level 9 gets +9 to attack. At that point, an attack against almost any foe has at least 50% chance of hitting. Our monk has a +1 quarterstaff, so he gets +10 with his first two hits. While Con is a common saving throw for PCs, not many monsters have a Con saving throw proficiency. So most creatures have at best a +5 to their con saving throw vs a DC of 15 to 17 at this point.
While the monk could burn up to 5 ki here out of its 9 at 9th level, it could also need to burn as few as 1 to get it to connect. The ki also recharges on a short rest. I am not saying that it is an automatic stun each time, but it is a very real possibility, and the consequences of being stunned are devastating for a solo monster.