If _some_ kind of life is possible in Venus, shouldn't Venus have it?
That's how life works on Earth. If it _can_ live somewhere, it's probably already there. We think that's related to how it can survive and adapt, so it's reasonable to assume other life would be similar in that regard.
Therefore, by this logic, if Venus is able to sustain any life, it should already be there.
I know it sounds weird. What if life never got there? Well, it had plenty of opportunity. The universe is old and lots of things happened in it already.
Maybe life is a late addition to the universe, and we're one of the first instances of it. Or maybe it's rare, and we're lucky. Both the "life is new" and "life is rare" ideas, however, paint a picture of Earth being a special case somehow. "We are a special case" is not a very satisfying answer though (from the scientific point of view).
> Therefore, by this logic, if Venus is able to sustain any life, it should already be there.
How do you know it's not? Venus is a much more difficult place for us to analyze than say, Mars.
Maybe it is, I don't know.
I was talking about your scenario of space-faring silicon-based life. If Venus can support this different life, then it should be already there.
It's hard enough to find signs of life similar to ours (something familiar, carbon, metabolism), so if there is some completely different life somewhere in the solar system, it's unlikely we would be able to recognize it at first glance. So unlikely, that is almost pointless to postulate its existence.
Therefore, our best shot is to look for life similar to ours (because we know its tells). I think that's where the idea of a habitability zone comes from. It's not excluding the possibility of life existing outside of the zone, it's just making it easier for us to look (because looking everywhere means considering things we can't possibly understand).
That idea has been slightly changed since we started to hypothesize life on moons like Europa and Enceladus though. They are outside the classical habitable zone, but they have other possible means for producing liquid water that don't rely on the heat of a close star (tidal friction and residual core heat).