All relevant points, but I don't think the first is fair: the longer you play, the more powerful you get, with no exceptions. Also, I don't think anyone's going to get rid of artifacts unless they trade or sell them to other players.
My primary reason for bringing this up is the fact that soon, they're going to be widely available and that some of the only occasionally useful ones will become obsolete. One example is Haldor. He has 3 artifacts, 2 of which are useful no matter what, one of which only helps in certain situations (Paper Doll). Even if he gets into a fight where the opponent can deal the Hexed effect on special damage, he's unlikely to take the turn needed to put the artifact on. (I probably wouldn't, anyway ) Essentially, it's just useless, even though it could save him at some point.
It's your third point that kind of goes along with what I'm saying - if the cap is at two artifacts, the hero's almost always just going to pick the one with the most generic, well-rounded power, rather than the interesting one-off artifacts that could come in handy in specific situations and build characterization.
For your fifth point, again, we're going to be able to buy certain artifacts at some point in the future, and it's unlikely to be stocking the most powerful, useful ones that come up in quests, and once everyone has two artifacts it's going to be sort of pointless despite all the interesting stuff that shows up, so heroes are just going to keep putting their gold in the same stuff - consumables and weapon upgrades. Which is fine, I just think the flavor of extra artifacts would be interesting.
Again, this is just for the sake of having something to discuss, not a serious suggestion. Interesting to see others' opinions.