People say "The Bitcoin network has an uptime of 99.9xx%", but what does that even mean? How is there a concept of uptime, on a network which has as it's sole primary activity, the posting of blocks to a blockchain? Can anyone make this make sense?

So, if there is a block every 10 minutes for a year, is that 100% uptime? What if the network slows down and there comes to be a block every 20 minutes, does that become only, say, 50% uptime? Of course it does not. But then that begs the question - what constitutes "uptime" on the Bitcoin network for purposes of that statistic? Does it mean that there is at least 1 node, somewhere, with at least 1 miner?

What would be necessary to happen, to declare the network to be "down"? I submit to you, that we characterize it wrongly. Wouldn't it make more sense to say that the Bitcoin network is always down, except for the very instant that a new block is posted?