While Goldhawk does not have a publisher, it did have a budget - the number of people who either paid in the Kickstarter or bought the game. That represents a finite number of zots that can be spent on the game, and all the zots were spent on bringing the game, 5 years in development, to reality. At this point Goldhawk had two choices.
1) Spend whatever zots it got from sales on Xenonauts.
2) Spend whatever zots it got from sales on developing a new game.
Spending what you have left on further development of the only game in your stable is an incredibly risky idea. You're betting the farm that more people than those who already bought the game will be attracted to it, and you're also betting that the people who bought the game will be invested adequately enough to by DLC for the game. This may be true when the game is hot off the presses, but as time passes, interest in the game itself wanes. Without DLC NOW NOW NOW from the moment the game is released, it's difficult to maintain interest. Take PayDay 2. It was release din August 2013. The first 2 for-pay DLC packs were in November, then January 2014, then May 2014, etc. etc. In addition to that were persistent free updates in-between each for-pay DLC pack. The budget and development structure of PayDay 2 was set up from the get-go to squirt out DLC very quickly (in development time) to maintain interest in the face of any competition (read: everything else). It's clear that landing Xenonauts was a monumental task which was budgeted and designed as "produce a complete game". Goldhawk has no publisher to ask money from to make DLC, nor it appears to have the infrastructure to rapidly support Xenonauts, given that a fair part of the original development team went bye-bye either during or after the game was released. So Goldhawk's survival depends on either developing a stable of games which can support it in lean times, or hyper developing a single game which supports everything that Goldhawk does. A stable of games spreads the risk of studio collapse due to one game failing. Which, if you were a responsible founder who likes game development more than he likes working in an accountants' office, would you choose?