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A chain is only as strong as the weakest link. Pithy, but true in this instance.

This is not true as long as you can link 2 POW blocks together.

Eg:
POW 99 ==> POS 100 ==> POW 101

Even if i came up with a new POS 100A block later, i will need to perform more work than POW101 to attack the chain.

ie:

POW 99 ==> POS 100 ==> POW 101
||
==> POS100A ==> POW 101A

But if POS100 was already broadcast first, POW miners are not going to sit around to build atop POS100A when they see it later.

This is no different than a POW block race where miners build atop the first block they see.
Ask yourself, Why don't they switch?
Because if i own any hashpower < 50%, by mining atop the later block, i sabotage myself by increasing the chance that my block will be orphaned as the rest of the network is likely to be mining atop the earlier block instead.

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Here is another attack angle for you, which isn't present in plain POW:

* Go slow

A majority stake holder can bring the entire chain to a crawl by withholding their POS blocks forever; so instead of a 10 minute block time, you'll have a 20 minute block time and there would be nothing anyone could do about it.

This is no different than saying that a large Chinese mining pool can withold blocks and slow the network to a crawl.
This is actually far more dangerous with POW because the largest pools have over 20% of hashing power and 66% of hashing power is within China.
If the Chinese gov shutdown all pools in China, blocks would slow to 30 minutes.

Contrast that to BTC distribution where the largest holder has less than 5% stake.

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Doesn't make any difference; there is still a constant probability of producing a block proportional to your stake, so the more stake you have, the more control you have to double spend.

See above reply. POW Miners will still build atop the first POS block they encounter as they have a real cost to mine ontop of a later block.