Well -2 on block and +7 for a short Karakusa leaves a lot of time to do something to interrupt. The natural reaction to blocking a Hayate is to punish so chances are they will press a button, at least the first few times anyway. That was part of my point however, once they start doing nothing after, LK Karakusa is the obvious attack.
27 frames sounds like a lot, but it really isn’t. You shouldn’t think too much about it in numbers (after all it isn’t combo theory), but more in actual match situations where, for example, you have discouraged ground advancement and have instilled the fear of the dash in them. Also, this is a counter-balance to the Hayate cancels some people love to do so much. If you ALLWAYS cancel a Hayate the surprise effect wears off. This also plays into what you said about it being parryable. Now, even though 11/60th of a second may be enough time to react to a HP Hayate and parry it, it is not practical. The only way someone can do this is by ONLY looking for that one thing. As a result they fall prey to a canceled Hayate into dash. If you allways release at the same tempo then parrying will become a rhythm thing and not a reaction thing. Avoiding that is another story altogether.
As for an opportunity. Take any matchup where your opponent is preemptively sticking out a move hoping to punish your dash. You can
A) wait a split second THEN dash
B) Hayate (possibly EX) into the fray hope to beat their move during rcovery or startup of their next move.
C) Jump for no reason
D) Not do anything and let them come to you
E) Use another normal to try and beat theirs
F) Hold a Hayate for half a second and either beat one of their attacks or be in for a guessing game with higher payoff than where you were at before
From my experience, in a neutral situations Makoto is usually being defended against rather than being attacked.