I think this hypothetical scenario is unrealistic. The cost of obtaining 51% control is so high that such an investment would be highly risky, given price volatility, especially if the plan is to implement a controversial monopoly which could cause the price to collapse.

As long as the government continues to control its own fiat currency, there's no need for it to hijack bitcoin in the near future. If some misguided policy folks thought it would be a good idea after bitcoin became hugely popular, then there would be a public outcry which would be a costly risk to any politician.

What is more realistic is that a government could hijack bitcoin in order to introduce massive volatility and cause panic selling, in an effort to undermine crypto currencies. However, we have already seen bitcoin survive a massive crash, so I don't know if that would work anyway.