You cannot say never about such things, as we have so little information to go
on Earth's (and the galaxy's) deep past.

What if we are the result of that alien "conquest" ages ago?

I am mostly playing devil's advocate here, just to show that saying aliens
have never visited Earth in our planet's 5 billion year history without solid
evidence (or lack thereof in this case) is quite premature.

Plus I keep getting this feeling from statements such as yours and from
others in recent years that the tone towards ETI has gone from wonder
and anticipation regarding other intelligent beings in the galaxy and the
positive potenitials for us from their discovery to fear that there might
be other societies out there, ones that could easily bring us down or
destroy us outright.

I could just as easily contend that if there were a species in the galaxy
that was bent on destroying all "competitors" that it would have done
so by now and we would not be here, so in fact there are no hostile
species in the galaxy.

Maybe WE are the potentially hostile invaders of the Milky Way and
all other life forms had best fear us. I can buy this one at our current
state of progress.

Of course it is a very big galaxy at 100,000 light years across and
containing 400 billion star systems. If most of those stars have a
collection of planets, it would take a long time for a society to
investigate them all. Plus it is also likely that they may not bother
with every system or want to keep moving on indefinitely.

And who says that Earth's environment is compatible with their
biologies? They might prefer to float around in Jupiter's atmosphere,
for example.