> On the one hand, the conditions under which some lifeforms are thriving here on Earth are already pretty extreme.
Not really. They are all within +/-5km of the planet surface, live under 1g of gravity, as a rule, are not exposed to X-rays, gamma rays, cosmic rays, strong magnetic fields or nuclear radiation, or extreme temperatures. There's nowhere on Earth that receives more than about average solar radiation or gets much above 100C where life survives.
Life generally has access to ample oxygen, though a few anaerobic organisms exist. They're all DNA-based, have access to ample amino acids, aren't bathed in thousands of completely toxic chemicals, have liquid water (and not, e.g. liquid methane). The list goes on and on.
Yeah, there are a few extremophiles, but they are by far the minority, and are clearly adapted incrementally from other organisms that evolved in non-extremophile environments. In essence, there's a large reservoir of easy-pickins living that generates biodiversity to cross over into extremophile environments.
Now imagine 5x the gravity, 100-1000x the solar irradiance, the lack of magnetic field (and thus direct exposure to solar winds), no atmosphere, or atmosphere entirely composed of a toxic gas, extreme exposure to X-rays, etc. The universe is full of many places where Earth life, even extremophiles, would have no chance of survival. And such places don't have reservoirs of biodiversity from which to adapt to these conditions. They're just dead.