Oh, I understand caution. To use the word "cautious" with my employer would be to offer an understatement that would make God fall over in a faint. We're just now getting out of IE8. My employer is also the 800-lb gorilla in our economic niche, so can "set the rules" to a certain extent. Even so, we are being dragged into IE10 (yes, that's right) and Office 2013 because when you fall behind you start running into problems. Not only technically, but also PR-wise.
"Uh, sorry, Bob, I can't access your website because it keeps telling me to upgrade to a modern browser."
In the case of DOCX, the format has been around for about 5 years. Generally, A company that doesn't want to be behind the curve has about a year before clients start to give you the stink-eye for not being up to date. I make this statement having been on both sides of this equation with multiple types of software.
While I'll grant that with agents, it's a "seller's market", if an agent bounces my attachment because they can't open DOCX, I am absolutely going to do a WTF. If they say they won't accept DOCX because of known issues, that's different. For the record, there aren't any signigicant issues, and these days Microsoft Updates fix those in reasonable time.
Not using DOCX does not mean you're behind the curve, it means you hate DOCX, and probably hae the latest versions of Word. An amazing number of people hate both. DOCX hs many problems, and it use is not nearly as widespread as you seem to think.
Publishing may be big business, but it is NOT like most other big businesses in the need to use the most up to date software. Many agents and editors still use Word 2003, and even many of those with the latest versions of Word may be able to open DOCX, but it does not mean they want to do so.
Many, too, use a Mac, and don't even have Word on it. And, frankly, I've never had an editor anywhere request DOCX. Not one. Every last one has request Word DOC, or RTF. I don't care how long DOCX has been around, many don't like it, and many have issues with it. Microsoft updates are fine, but if I have to wait for an update, I'm not using that file format, and many still do have serious issues with DOCX.
The biggest issue is one that Microsoft can't fix. It's that many agents an editors don't like DOCX, and see no need for it. I see no need for it, either.
In publishing, it's agents and editors who raise the stink, and the rest of us need to go along with what they want.