Honestly while I see the appeal of getting story companions later on in a game, I feel like the reason it's not done anymore is the fact that characters who come in later cannot get the same amount of time and attention from the character as companions introduced in act one. I literally do not think there is any way to alleviate this problem. If act one of the game is 10 hours long, then introducing a character in act 2 means the player has 10 hours less time to get to know that character and get invested in them. The player has had ten hours to settle into the party members they already have and get used to using them and understanding what they can do. It also limits a new players flexibility if not done right. In PoE1 there's a character that potentially you might not meet until act 3 of that game, since he's in an area that you may not find a reason to visit until then. Every time I've played through since then I make it a point to go get him as soon as possible becasue I don't want to miss out on having him around.
