I don't see what is so hard about switching the vast majority of common office computers
The basic problem is that you are likely throwing away decades of experience on average per user. For example, I have been using MS Word since the late 1980s (ok, I am older than average; average is probably about 20-25 years). Yes, it has changed over time, but many concepts remain the same and I've had continuous re-training. Could I use another word processor? Yes. Will I be as effective using it? No, not for 4-5 years of using the new program. Knowing the new way to use styles, format pages, etc, just takes time to learn. Of course, I could make basic documents using LibreOffice or whatever alternative program is proposed, but it just wouldn't be as good or as fast. Learning to use a new program isn't the primary task... getting a document written is.
Excel is probably a much bigger problem than Word. There are a very large number of power-users of that program who just won't be as effective using other programs. It is used extensively in the corporate world and likely in governments as well.
When you're talking about an OS, it is also file management, etc. These are probably faster to learn as most users probably don't do sophisticated things with their file organization, etc.
If you change all at once, you are asking for a lot of user complaints, a lot of confused people stumped on "obvious" problems. I am not arguing that it shouldn't be a goal to use open source, I am just pointing out some problems. For anyone based outside the US, I would think that migrating off of US-based products for anything critical would become essential given the way the US has recently abused sanctions (e.g. sanctioning members of the International Criminal Court). The US just has far too much leverage.