As Arez said. I am not aware of any servers TFP operates. As such the cost of servers does not come into play as an incentive to shorten the life of the game. Once the game is published there will be no running costs (AFAIK) except for costs they might do without being forced, aka bug fixes or dlcs. Instead successful long-running games produce the "long tail" of sales which actually every developer wants to generate a safety net of income for bad times. Without a good reason no developer would endanger that source of income IMHO.
The only possible motives for shortening the lifespan would be:
* to sell more copies of a successor-game and that they would not like people to stay on playing the old game. A pretty weak motive as the fun people have with the current game would influence their incentive to buy the successor-game negatively. And I doubt any (happy) long-time player of a game would not at least buy the follow-up game to try it out so I doubt this would really generate any additional sales from that group. If at all such a move would have to be done in a patch or dlc AFTER the successor-game was released, not before even the original game is even released and a successor-game isn't even on the far horizon.
* or if the successor-game were a subscription game or micro-transaction game where the income is directly linked with the number of actual ative players at the moment. This only works if the successor-game of 7D2D would move to a central server/massively-multiplayer modding-unfriendly system. Such a game would not draw the same crowd as this game, it hardly could be called a successor game to 7D2D
Also that this game is so modding-friendly makes it almost impossible to make the game undesirable for veteran players. A player can always install a mod that removes unfun changes or replace changes with something that mimics previous features. It usually is not possible or at least very laborious to implement exact copies of features of old alphas, but it always is possible to make very similar features. And if someone just doesn't like the longer duration of the early-game even the normal options are enough to change this.