I took a stab at this yesterday. I think the main point here is that those who have been given the gift of salvation and then descend into apostasy are really, really in trouble.
I wonder if the word "imposible" here is meant to be taken literally. It would seem to be shutting out a group of people from God's mercy and seems to run counter to other stories (like the prodigal son). I guess it depends on what "apostasy" means. If apostasy is "the sin against the Holy Spirit" then it is re-affirmed that those who do it can not be forgiven. I guess we should steer clear of apostasy then.
You make an interesting argument that those who have "tasted the goodness" can never be forgiven, therefore they must not be able to fall. I don't buy it. Certainly if you start with the idea that those chosen can not be lost, you reach a conundrum here which must be solved by making falling impossible.
If you start with the idea that some will persevere and some will not, this becomes a stark warning that falling away is irrepairable. SD