To me, if you obtain the bike, it's no longer a "grail." To me, a "grail" is something you long for, but it's elusive, you never quite obtain it, for whatever reason.
+1 Plus tastes and wants are always changing.
Remember that the grail was a carpenter's cup and not a King's cup.
A bicycle is a mode of transportation first and foremost, so if it is sitting collecting dust like a trophy then it is pretty, but mostly useless. (the exception is as an investment) Plus at the highest level they are somewhat generic. What makes one more coveted than the other is strictly up to the viewer. Think of supermodels as an analogy. At that level they are all super attractive. But what makes one more coveted over the others is strictly up to the viewer. Blonde is my least favorite of the hair colors. So the blondes are a notch below in my book. In transportation, white and black are my least favorite colors, particularly white. White screams "fleet" and black is dirty minutes after it's washed. So give me a car or bike with some color NO matter what the brand is. But that's just me.
You could also argue that there is a "grail" for every level or type of bike. For instance, according to this forum, a/an UO8 should be considered a grail bike. Why, because it is considered one of, if not the best entry/mid level bike of all time and everyone should own one. I'm sure the same could be said for most entry/mid level french bikes my 72 Lejeune TDF being one. Think about if you can only have one bike to ride for the rest of your life. Is it really going to be the Italian race bike with the corn cob on the back and the ultra skinny tires on it? I don't think so. But if you are dreaming and wishing, that is a different story.
On a side note, did the Italians only make good high end bikes? You never hear people talking about their entry levels like the french ones.