@KozumoJi
That’s what I was thinking of when first watching it.

@Phoenixflambe
Duh, the three races are still at odds with each other and friendship hasn’t been restored yet. Argyle probably thought the three races would’ve been reunited already when Sunny used the contraption, or he didn’t even know friendship needed to be brought back for the crystals to work. He may have just thought the crystals alone would bring magic back just like his daughter did.

@Wayneponeth
That but it never activated even when put on.

@Phoenixflambe
Meta wise it was for the 3 mares to have a tense action scene on the lighthouse, including Izzy almost falling from it. In-universe lore wise, it’s probably because Argyle believed one day when everypony finally came to peace with each other Sunny would’ve figured out the truth by then, so she and her friends would calmly place the crystals on the lighthouse without any complications. Simply put, Argyle didn’t expect the lighthouse to be destroyed.

@Wayneponeth
What was the point of that contraption in the first place? Did someone had to go through all the trouble creating red herrings?

Honestly I initially didn’t like the fact that there is a third crystal. I thought it was really as easy as Sunny simply forgetting one more crystal and not the fact that friendship was lost among ponies. Thankfully the third crystal still didn’t work and it is truly because the races are so divided that magic is gone. The crystals only light up after the three races reunited. And even better that contraption in the lighthouse isn’t the key to the solution either. The picture frame with Sunny and Argyle in it is actually the real important thing they have to put together.

@KozumoJi
I knew there’s a third crystal, didn’t know it was at the beginning that’s the lighthouse.

At this point in the movie, I immediately knew that this was the final crystal Sunny had been missing all along and she had never even bother checking the underside of her dad’s present beforehoof