achieving a viable colony
For instance, it is now speculated that we will have the capacity to detect earth-size planets a few light years away some decades from now given the current rate of astronomical development. Let us say we have the ability to detect an earth type planet 1 light year away. Then ... the detection of an earth type planet 40 light years away will require a 1600 fold increase in the signal capturing capability of whatever apparatus is used. That is, we must build something 1600 times as big (areawise) as we then possess.
Clearly, it is more reasonable to build a 1600x receiver and get instant results than to build a probe and its enormous propulsion system and wait as long as 10,000 years to get a reply (that the probe has failed ;o).
Suitable systems for colonization
We can reasonably assume that in the coming centuries we shall be able to acquire vast quantities of information about our nearest stellar neighbors ... and ... if one is deemed suitable for colonization ... that is what will happen in due course.
What does "suitable" mean?
First, the solar parameter must be met. The star at the center of the candidate system must be the appropriate mass, luminosity, type, age ... to sustain life for some billions of years. If it were suitable for sustaining life for only tens of millions of years, it would be OK for a visit but you wouldn't want to set up permanent shop ... and "just visiting" would require some sort of propulsion system unlike anything yet contemplated, i.e. a sports car to get there real quick and inexpensively so that the project is moderately worthwhile. After all, why just visit a place when we can acquire vast quantities of information about without even going ... by just using the 1600x to examine it?
Suitable also means that the candidate system must have planets in stable orbits around that star at reasonable distances from it. We don't necessarily need an earthlike planet. These can be "made" (terra-formed). We just need something to land on which provides earthlike gravity. A Jupiter size planet with nothing else would be inappropriate as well as a system with just a bunch of little, cheesy asteroids. We need a bonified, rock-solid planet to walk on ... and throw in some water as well.
Having found this ... we're ready to "go for it".
First step ... Send in the Clowns
Colonization is not to be accomplished in one step with one huge "Noah's ark". It would be more like a military operation with an advanced landing followed by a steady build up leading to independent conduct.
First off is the lead probe. Your "point man" is sent a few years ahead of anything else so that it can detect the disposition of the space into which those following will be traveling. If there is anything bad, those which follow can potentially change direction or, at worst, turn to an alternate destination. There will be many such unmanned lead probes so that something is always functional out ahead of ...
The main body follows in the form of a few very large ships and many smaller ships of varying sizes such that if anything bad happens to one, the others will take up the survivors. They will be spread out in space over perhaps as much as a month's travel time, i.e. if you want to get to another ship it might take a month to get there in a smaller craft.
The larger ships will function as temporary places to live until cities are constructed underground on the new home planet (which presumably needs to be terra-formed before moving onto the surface).
And all the rest ...
As decades pass, the new colony will always be in communication with the home planet even though signal transmission may take years. New "flotilas" will arrive from there every few decades bringing an ever increasing supply of "hands" to make a new world. In time, the new colony will become self-sufficient and will drift away from the home planet within the galaxy as all stars do in time.
Isn't the foregoing reasonable?
What is to keep this from happening? And would it not also be reasonable to suppose that it happens many times on each planet and for that matter, on every colonized planet the achieves independence? Would not such "home planets" send out new colonization expeditions every few tens of thousands of years? What's to stop them from doing so?
It seems that professional scientists (most of them but not all) have a limited view of future possibilities and probabilities based on their own professed beliefs, i.e. that space travel to other solar systems is possible and that there are "others" out there besides us. It is clear that if their own opinions are true, it necessarily follows that ... statistically speaking ... the galaxy ought to be already completely explored.
And the time required for such total colonization (originating from a single planet) is on the order of 5-50 million years as I have given it elsewhere. If many more join in at the beginning of colonization, it could take much less. So, as Fermi said ... for the same reasons given here ...