I think you're roughly correct. 49% doesn't automatically become 51% purely through the PoS process. But if the owner of 49% of the coin goes out and buys an additional 2% on the market, then they have a majority of the coins and it's all over.

It's not guaranteed to happen, but does seem plausible that some single entity could end up with 51% sometime in the future. Or a group could band together to form a majority and steal everyone else's money.

It does become harder to prevent a coup over time. As more coins are issued the cost to buy a 1% stake increases but the cost to maintain a 1% stake does not.