All billable hours aren't created equally. If I can automate the work that I currently hire fresh grads to do - the work of actually drafting up contract agreements for review and wills - but price my services just under the prices offered by other firms then I can make a killing.
You can purchase a hand carved wooden table for your living room and some people still actively manufacture these "bespoke" items, however most folks shop at IKEA and chose between one of a dozen styles available for a dining room table. These two products are mostly functionally identical in terms of supporting food and weathering the years (if you go by depreciation the IKEA table probably loses value slower than a hand carved table - and you can replace it pretty much at will). There will likely always be contract lawyers around to sort out large M&A agreements and other high priced transactions, but wills and simple employment contracts - delivering these in a correct format (i.e. not just a download a sample contract and fill in names possibly resulting in something unenforceable) will be a game changer.
Taxes are extremely complicated - but they got that way by design. The US Tax code is the result of one of the most insane applications of regulatory capture in the world and it's there to allow Intuit to continue printing money at the expense of the tax payer - simplifying those (like for real - not the 2017 version) might not end up happening for decades as there is a ton of money entrenched - so that seems like an incredibly poor part of the market to initially target.
Contract law though, it's not simple but it's so utterly clear and actually codifying it would likely greatly aide law makers in discovering and resolving any existing loopholes and might even provide some useful insight into how the US could better handle the question of employee vs. contractor by enumerating rights and privileges in a clearer format.
And wills, pretty much everyone seeks independent legal council to write one of these at some point - it is highly advisable depending on where you live (in Canada a lack of a clear will causes assets to be tied up by the government for an extended period of time) so you could make a lot of money edging the market out there in particular.