Signals are an essential aspect of human behavior. I don't think trying to eliminate signaling is a productive goal.
You yourself brought up the "social contract" of marriage. A contract is not very powerful if there aren't consequences for breaking it. So that's one crucial signal: we are committed enough to this relationship to spend money on something which is only useful as a signal.
There's also the signals of "my mate is wealthy enough to buy me an expensive diamond" and "he cares enough about me to spend a large sum of money on something which has no useful purpose besides signaling his love for me." Simply wishing that these signals didn't exist won't make them go away.
Useful things make poor signals because you might buy them without any intention of signaling anything. That's why glasses are a worse signal than jewelry, even though they cost just as much: you might be spending money on glasses because you need to see, not because you have money to burn.
No, we try to make the case for the waste and stupidity, and when a group still can not come to terms with it, we shame them.
If something external, such as an expensive ring, is to keep the relationship afloat, I have hard time seeing a point of living and sharing your life with someone.
Why do we need signals? Why does the social contract of marriage need an expensive down payment on essentially nothing? These are things modern cultures should be distancing themselves from. The down payment should be for a house or apartment, or just anything useful.