Either you aren't listening or aren't thinking this through. The 14 years simply provides more accuracy to the long-ago measurement. Even if you think that the events in the Large Magellanic Cloud aren't all of 150,000 years old, they still happened a long time ago by any standard. The deceleration measurement is happening over that long time. I say it's 150,000 years, and while your mileage may vary <grin>, it's still going to be a long time.
So pick a number! Tell me how long ago you think SN1987a occurred, that is, tell me how many years you think the light's been travelling to get to us. That number (not 14 years) is the time over which the "decay" is being measured.
So how long ago was it? I dare you to guess.
Since you can't measure any decay in 14 years then it doesn't decay.
That brings up another argument. With the kind of deceleration you would need, and with the acccuracy to which we can measure the speed of light in the laboratory, you unquestionably would be able to measure it unambiguously over a span of 14 years.