It's clear what was meant was "cost is no object, so it's not an issue" - law firms probably pay more per-employee for things like travel expenses or coffee each year than for MS Office licenses.
I have seen that MS Office suite has become a hammer used to drive many non-nails in various businesses, and would like to see people using more open standards. But I totally understand that for many use cases it's good enough and that people consider the cost to be not particularly significant
An individual business license of MS office 365 costs about 10 dollars a month per user. So even at minimum wage fully loaded costs this is approximately an hour. It's more like a minute or two for a lawyer.
Do you think having access to a tool with the least communication friction saves you 2 minutes/month of your lawyers time? 30 min of your junior marketing person? This isn't a hard call to make usually.
I'm more concerned about the people who are subject to the laws being written and (imo) deserve to be able to read them for free.
As for lawyers, just because they can afford to use expensive software doesn't mean they should. I can afford to write software in Word, but I don't plan to try that any time soon.
Also, I think people seriously overstate how "easy" it is to use Word. I have spent litteral days of my life in classes that taught me how to use Word. I have spent hours relearning MS Office interfaces as they've changed over the years. I'm pretty sure I picked up GitHub-flavored Markdown in less than 15 minutes.
Your first point is valid but quite separable, I think access is an important issue.
My point about the cost is that the network effects of the tool here are more important than the tool itself. You might be able to have a better local workflow using other tools, but if everyone you collaborate/work with doesn't use them then the cost of communication is far higher than any savings.
Realistically, legal work isn't much like coding. Although lawyers do spend time editing documents that isn't where the real time is. For that reason I'm not convinced that changing to a putative VCS workflow would have enough efficiency impact to pay for the training etc. except on a very long term. This sort of thing makes it very difficult to win out against network effects.
This is much the same reason that excel is used for a lot of things where better tools exist. The combination of network effects and having better things to do than learn a new approach and toolchain are hard to shift.
Lawyers can afford free software too.