I have strong views on this subject, as developed in 30+ years of practice as a business lawyer in Silicon Valley (they are articulated here: http://grellas.com/articles.html).

The best legal writing has the same goal as an other form of expository writing - that is, in general, to communicate clearly and concisely with an intended audience.

Legal writing can (and often does) also carry with it the goal of persuading a target reader in some fashion and in this sense is best deployed using not only clarity and concision but also the tools of rhetoric, which can often be quite refined and which can but most often do not include obfuscatory tactics. The shyster model of lawyering says play for advantage to the disregard of truth. Carried to the extreme, it can include amazing tricks aimed at dissembling and hucksterism but it does not necessarily involve using language that is badly written - indeed, the finest forms of flim-flammery come from top legal echelons, where very high-priced lawyers are often paid to craft misleading arguments that will benefit their clients (including the government).

The so-called Darwinian theory alluded to in the OP, however, is not a good explanation for shoddy legal writing or, indeed, for legal writing that is not shoddy but rather unduly complex.

Many years ago, when much of law consisted of small-town practice and when very little information was inter-linked or easily accessible, hack lawyers would draft contracts using case digests and or similar third-rate resources (often not even written by lawyers) for guidance of what was or was not acceptable to the courts in matters that might eventually be litigated. That sort of practice got the defenestration treatment several decades ago and survives today primarily in narrow specialty areas (such as certain forms of insurance contracts) where "tested" language is sometimes used simply because it has been upheld by the courts.

In the vast portion of the business-law field, however, contracts are generally drafted with the aim of making them clear and unambiguous and a very good way to get fired at a large law firm (or any other firm that maintains high standards) is to put together a shoddy contract that shows you have no ability to state things clearly and unambiguously.

Of course, in a negotiation over a contract, one might try to slip in some "snake oil" language which has as its goal the aim of putting something over on someone. In such cases, a deliberate choice is often made to use ambiguous language in order to further that goal. A good lawyer on the other side will usually pick up on this and come back with cleaner language, but many times people miss the subtleties and someone gets had. This scenario, however, doe not define any significant portion of the contract drafting that is done but rather constitutes the exceptional case in customized deals that are negotiated in private and usually kept private. In the event such a clause is litigated, believe me, in modern law, it will not serve as a model for future drafting.

By far the greatest reason why legal drafting can become hard to read has to do with the complexity of the subject matter and the nature of the deal and not with any goal of obfuscation. For example, one can indemnify (hold harmless) another party with simple contract language that says, "I will indemnify and hold you harmless from all damages resulting from x." That same simple clause can be expanded to fill two or more pages of a single-spaced legal contract if enough is at stake and if the parties are motivated to protect their interests with a detailed contract that covers all the possibilities. In the example just cited, for instance, you can add elaborate clauses describing the exact types of claims against which indemnity will be provided and you can specify the types of claims against which it will not be provided. You can include notice requirements and specify consequences if they are not met. You can include caps and limits of various sorts. You can state that indemnity must be made when a claim is first asserted or you can state that it will only apply if there is a final judgment. These sorts of details can be replicated in great number, depending on how important it is to the parties to have them in the deal. This can easily lead to a fairly complex clause with a lot of legal terms of art in it, but the overriding goal is to keep it clear and unambiguous, not to obfuscate. This sort of contract language gets drafted every day and does not normally get litigated. It is an example of lawyering at or near its best, provided a deal warrants it. If not, you can always say "I will indemnify you from all claims" and leave it at that.

Thus, the point made in the OP is either wrong or at least vastly overstated as applies to modern business lawyering.