There is not a simple answer to that question, because it how you choose to look at it. Yes the game as it is of now is done, season 1 is over and IO told the story they wanted with season and all maps from this season have been released. However season 2 is coming and will continue the narrative. Season is still going on and will for some time, the game keeps getting Elusive targets, people keep creating contracts and new Escalations are being added that helps with the longevity of season 1.
Yes and no. Don't expect to see a big change in style or graphics, like we did with the gap between Blood Money or Absolution. Those two are two different games with their own narrative. Where HITMAN 2016 design is inspired of a TV-Show and there for an ongoing experience with a narrative that evolves as more seasons are released.
That doesn't mean that there isn't going to gameplay changes or slightly upgraded graphics. There will be new locations to vist, new targets and other kinds of content that does not exist in season one.
Well if you don't mind the season format then you don't mind DLC, season is basically 5 different DLC on top of a base game. DLC does not equal bad or worse, it's all about how it's used. You can look at it like this HITMAN Season One is basically World of Warcraft base game. Then we get a new Expansion Pack the Burning Crusade (season 2), later down the line Wrath of the Lich King is released (Season 3) and so on. Each expansion to the game have come with it's share of changes to the base game. World Of Warcraft have been an ongoing and developing experience since 2004 up until now. A expansion is more or less a DLC in itself.
Well what is wrong with that? do you expect to every season for free? each season is developed on it's own and can in terms of development be viewed as a game in itself. If you buy Game of Thrones season 1, do you expect to get season 2,3,4,5,6 and 7 for free? This same principle applies here.
But yes games in later times have been very anti consumer, if we look at games like Battlefront, Battlefield, Call of Duty, Assassin's Creed Unity. All of these games chucks up the base game and creates a "season pass". In this these examples you end you paying double the price for the same game. That can't be said about HITMAN. You could buy the hole game for around 45 eur or buy each episode as they where released. IO and Squre-Enix is not giving you extra things to purchase, they game you a model where you are free to choose how you want to pay. give 12 eur for the base game, you have the chance to see if you like it before you commit to the rest of the game.