You are right in everything except that it’s absolutely not the superior option in OPs situation. A lawyer has to guarantee that a document is identical and recallable in its exact form even years from now and the only way to absolutely ensure this is using Office itself.
Furthermore, most law firms use and have to access other parties document management systems and the thick clients for these with all the features only run on Windows. Screwing around with WINE to get these maybe working isn’t worth the hassle for a time-poor lawyer with cases to work on.
Computers are just another tool in the box when it comes to your business and you should use the best version of it to get the job done. It’s simply only Windows in OPs case here that’s the viable option. It might be corruption or vendor lock in or whatever but there’s no point trying to fight it in this scenario.