There's also the thought that evolutionary adaptations are so tightly coupled with environmental landscape that it'd primarily work the other way around. For whatever defense mechanism we lack for that particular parasite, they lack anything to combat their own million-year-more-efficient predators (like our immune system). It is less about evolution moving forward and being more 'advanced', and more about evolution ensuring an efficient equilibrium with a crazy-dynamic environment. Jumping into a new environment after being conditioned for a very different one likely leads to death much much more often than an accidental advantage. You might not be able to even breathe were you transported a few hundred million years either forward or backward.
Depending on how far back you go, today's planetary environment is a very different place - in terms of chemical food, predatory tactics, biochemical efficiency, atmosphere, etc. Having to compete against organisms that are efficiently adapted in our current environment will almost certainly be immediately lethal for these revived organisms.
Not really. A single organism (outside of the human race) cannot call the full force of an entire 'environment' against another species. And species (outside of humans) must abide by natural laws of energy transfer - for every advance there is an energetic tradeoff. The more different the organism is the easier it would be to target with drugs that only target precisely those differences. The less different, the easier our own bodies would work on it. The more lethal it was, the less it would spread. The less lethal, the more time there would be to figure it out. Without bringing to bear the force of an environmental change (shifting the equilibrium or set-point), there is very little that could be biologically catastrophic for the human race.
That is not to say that a virulent plague would not be a bad thing for a lot of individuals, if not the human species - but it'd be pretty challenging to even design worse than already exist out there - HIV, ebola, marburg or smallpox.
Something equivalent in virality and lethality to smallpox could be catastrophic if you consider the geopolitical consequences as well as the initial biological impact.
A nonlinear system, like an epidemic, can have phase changes. Most epidemics don't make it very far, but we should still be nervous about that one which crosses the manifold into pandemic.
One should keep in mind, for example, that less than 1% of the Liberian population ever caught Ebola, but the consequences for the country have been serious and long-running.
Depending on how far back you go, today's planetary environment is a very different place - in terms of chemical food, predatory tactics, biochemical efficiency, atmosphere, etc. Having to compete against organisms that are efficiently adapted in our current environment will almost certainly be immediately lethal for these revived organisms.