Regarding whether it would be more responsible to pay down student loans/down payment/pretty much anything other than a diamond - absolutely.
But, if your goal is to demonstrate commitment to the betrothal - the Diamond ring does the job.
The Diamond has the additional (and important) aspect of being a demonstrable and physical artifact of the proposers intent. Plus, it's sparkly.
The vacation doesn't serve our purpose as well, because the proposer will get significant utility value out of the trip. But you are on the right track.
Indeed - a lot of diamond knockers haven't really looked at a nice diamond in the sunlight. They are true wonders of light refraction.
In your vacation example - it's impossible for the casual onlooker to know that the tan and relaxed demeanour came from a trip to Bora Bora, rather than a golfing trip to Arizona. The important point for the diamond wearer is the the long-term appearance of having found someone willing to waste money on a useless item.
A large part of the use of the diamond is the social proof - any casual observance of the behaviour of females comparing engagement rings. Yes, it is totally irrational. Yes, it is manipulated by marketing. Yes, it is what it is. If you go mental at every irrational (as far as the purchase goes) act caused by marketing controlling social norms, well, you'll go insane. Human nature is obsessed with status symbols - always has and always will - these impulses are there for marketers to interpret and direct. Always have and always will.
Yes, it is totally irrational. Yes, it is manipulated by marketing. Yes, it is what it is. If you go mental at every irrational (as far as the purchase goes) act caused by marketing controlling social norms, well, you'll go insane. Human nature is obsessed with status symbols - always has and always will - these impulses are there for marketers to interpret and direct.
+1 ... HN is full of engineers who like to think that we are superior because we are not susceptible (ha!) to this irrationality. But of course we live in the human, social world, and it's good to understand these things, even if we seek to push back against them.
But, if your goal is to demonstrate commitment to the betrothal - the Diamond ring does the job.
The Diamond has the additional (and important) aspect of being a demonstrable and physical artifact of the proposers intent. Plus, it's sparkly.
The vacation doesn't serve our purpose as well, because the proposer will get significant utility value out of the trip. But you are on the right track.