You're kinda missing my point. (If I even had a point, which is doubtful, but there you go.)
Let's start with the second. It's a convenient(ish) measurement for splitting up time on Earth. But what happens when we go to Mars, where there are 24 hours, 37 minutes, and 22 seconds in a day? That would just piss me right off. Or how about the moon, where one day lasts 29 days, 12 hours, and 44 minutes?
Instead of 9,192,631,770 periods of the...blah blah blah...of caesium-133 atom, why not make it 10 billion. (I'm not going to bother finding out the difference between caesium and hydrogen.) A second would then be eight percent longer. I doubt many people would even notice. World record holders might be a bit miffed, admittedly, but simply convert the records and nothing would really change. We could even switch to a decimal time system, and then no specific world would be "special" with regards to time. (We're going to have to switch at some point in the future. Or at the very least every world will probably end up devising its own timescale to measure its sidereal rotation period - it'll be like the train network all over again. Will we stick to Earth times when in deep space? I don't ever recall hearing "Captain's Log, 13th of June..." after all. ;)
Now let's look at the metre. If it were the distance light travelled in 300,000,000ths of a second instead of 299,792,458ths (which is still a bit arbitrary for me), it would be around 88 millimetres longer than it is today (with the new second). Personally I'd rather opt for 100,000,000ths of a second, which, ironically, would put it closer to the foot (actually just over 14 inches) than the metre. Still usable for human scales, but slightly less arbitrary.
My point (yes, there really was one) was this: Why not redefine SI measurements to be less arbitrary instead of forcing what we currently have (which we only have for historical reasons) to fit some observed natural/physical phenomenon.
And before you claim such changes would be impossible... every country on the planet has changed some or all of their measurements at some point in the past.