If the diamond's function is to be long-lasting, beautiful, and satisfy the expectations in certain cultures of a gemstone to symbolise one has committed to marry a certain person, then moissanite is a cheaper alternative to an actual diamond which is just as good (or arguably better, in terms of beauty). The same goes for a Rolex. If its function is to look good and be a dependable timekeeping instrument, then many other watchmakers can be a perfectly valid and less expensive option.

However, if a diamond's only function in an engagement ring is to prove that one's partner has spent a large amount of money to commit to marriage, then of course moissanite would indeed be analogous to a "fake Rolex". But then the whole idea of finding an alternative to diamond engagement rings due to the disproportionate price of diamonds would be to beg the question — there cannot be an alternative, for the very purpose of an engagement ring would be to spend a stupid amount of money.

It could easily be argued that your Seiko makes a better watch than a Rolex.

What people are looking for and value in the product will make the argument more or less compelling.

One of those things they might value is the similarity to some other well-known, valued product.

Moissanite apparently looks like a diamond but better. I could say the same about a number of watches in comparison to Rolex.