Worried about dynamic checkpoints.

I understand the concept behind a moving checkpoint that prevents 10 block reorgs, and I agree that a 10 block reorg is definitely an attack in practice and should be stopped if possible. Proof of work seems useful for deciding the next block but there is no legitimate case that I can think of where a 9 block reorg is part of the normal "concensus" process. But I'm concerned about two things.

First of all, to do a 10 block reorg you need a lot of hash power. I would argue that if you can get that much power you can also get 51%. If you get 51% wouldn't you also be able to do 9 block reorgs with empty blocks all the time effectively making the currency useless? What's the threat model here? What kind of attacker is the reorg protection protecting us from? How much hash power would they need to have? Is the only point of these checkpoints that if a TX is 10 blocks deep we know for sure that it can never been changed but they don't protect from someone messing up the currency in general? I guess I sort of answered my own question here but confirm/deny please. :-)

Secondly, what happens if half the network sees a 9 block reorg and the other half a 10 block reorg? Wouldn't half the network reorg and the other half ignore the longer chain? Am I missing something? Isn't this a race condition? Most importantly wouldn't this be something that can be pulled easily be someone who has the resources to do 10 block reorgs?

Thanks.

submitted by /u/Tritonio
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source https://www.reddit.com/r/btc/comments/a1gg5y/worried_about_dynamic_checkpoints/