It's kinda like taking over a condo building on a much larger scale: the people you buy out first can charge a premium; by the end, you set the terms.
The Ethereum everyone talks about today is the fork, due to the Ethereum Foundation which owns the trademark leading the fork.
The Ethereum blockchain with the unaltered history is called Ethereum Classic
https://en.wikipedia.org/wiki/Ethereum_Classic#The_DAO_bailo...
Ethereum devs were unable to create a legitimate transaction reverting the DAO funds because they do not have access to the hackers' private key. The reversion was done with a "surgical state change" hardcoded into the client itself.
The price goes up, you've just made it more expensive for yourself to take over the network.
If you were to attempt to take over you'd be better to do so in silence. However, it would be hard to hide that kind of control and wealth when every information on the network is publicly available.
If you're intentionally trying to take things down, sellers have a huge incentive to not be left in the 49% who hold something that's now lost its value - as you say, "as it's become evident you've taken over the network the value of the network is essentially worthless." I think you could get the value to go to worthless well before actually hitting 51% on intent alone, if you're a big enough power.