> Um, because it's the prime meridian and that's how UTC is defined?
That's an explanation of how it is, not why we should care to preserve it.
The definitions of hours minutes and seconds have changed before, and in recent history.
> Which is why I was careful to specify mean solar noon.
And "mean solar noon" is meaningless to people's lives. Even in the areas where time zones do follow meridians and not country borders that are many minutes off.
> The definitions of hours minutes and seconds have changed before, and in recent history.
In terms of what physical process we use to set the standard, yes. But those very changes were made to try to preserve the same time periods that were important to humans. In other words, to not change what hours, minutes, and seconds mean intuitively to us humans as we go about our daily lives.