Major The syntax of the Property and the NavigationProperty element is almost identical. The only difference is that the Type attribute of a Property MUST NOT specify an entity type, while the NavigationProperty MUST specify an entity type (or a collection of...).
Although Properties and Navigation Properties share some of the same attributes (there are attributes like Partner and ContainsTarget that only apply to NavigationProperty, and facets that only apply to properties), Properties and Navigation Properties are conceptually very different. Where/how they are used in the URI and payload are different, the annotations applied to each are likely to be very different, etc.
Rather than simplifying the model, in this case trying to use a single element to represent these two very different things obscures those differences, making the model harder to understand.
I'm not in favor of making things implicit; i.e., of having to understand and look at the type of the property in order to figure out that it is a navigation property (and can be used in things like $select) versus a simple property. Since there are real differences between how and where properties versus navigation properties are used, it is simpler to make it explicit by having separate elements.
Also, note that the partner attribute is optional for navigation properties (i.e., a property may be unidirectional).