I think you are mixing different issues together.
Many, if not most, "authentic" designer and niche colognes include a lot of synthetic materials. Aside from price issues and current tastes, it should cut down on the environmental wear and tear that harvesting from scarce or endangered plants and animals can cause. Lavender isn't even that scarce, but I've read that the demand far, far outstretches the potential supply. Some of the synthetic aromachemicals aren't all that cheap, at least not when they are first being introduced.
As far as "oil" vs. "alcohol" as a carrier solvent, the physical chemistry is different enough that that changes which of the available aromatic materials can be used, and how concentrated they can get. It also changes how well the perfume adheres to the skin and how fast it diffuses into the air. Both in turn will change the smell and development, even among similar styles. BTW, some original niche fragrances are sold in perfume oil form.
The knockoff "impressions" of more famous fragrances - whether sold as a perfume oil or as an alcohol-based cologne spray - normally aren't going to smell much like the original fragrances, even if the bottle cap smells the same. If you want to compare scents before laying out the big bucks, ask for, trade for, or buy samples of the real fragrances you are interested in. There is no reliable substitute.