Office 2016 is going EOL in two years.
That's from Microsoft themselves. They do not hide these facts or make it hard to find.
I find it mindboggling that a simple program like text processors have to be continually updated for decades. Just program it right once for god sakes.
Your assumption that a word processor is a simple program is something you might want to consider, at a low level handling text rendering in a word processor is highly complex work. Besides text encodings regularly evolving and changing over the years especially in the pre-UTF-8 world (but even with Unicode), there's also the reality that security threats evolve over time, and once threats are discovered old code that once seemed fine becomes insecure and dangerous. In computing the reality is that there's constant change driven by supporting a regularly changing computing environment, security fixes, bug fixes, increased computing power permitting new features that are then implemented and new ideas appearing, et al. Software will always be changing, that's the way things are, there's good reasons for this. Trying to oppose that reality with an unrealistic model that doesn't account for the causes of change just leaves you misunderstanding the way the industry works.