Fatshark is no longer a 15 man studio, it has 90+ employees, like other big studios, it is capable of developing, maintaining, and continuing support on multiple titles.
When a project is completed, a big chunk of the team moves on, since there skills are no longer required, a smaller team remains, that works on bugs, updates, and new content. Why? because keeping 90 people tied to a project that is past development phase is incredibly expensive, and considering the big sales take place on release, it is not sustainable. Also the tools are already there, and adding stuff with them, requires a lot less people.
As far as Vermintide is concerned, Darktide was never meant to replace it, it was meant to do the same thing WH40k did, appeal to people that like guns over swords. Why? to give fatshark another revenue stream, so it can keep growing, and make more games.
Why is this good for Vermintide? because it means bigger sequels, and more content, because there is more money flowing in, wich leads to more hiring, wich means more people working on a sequel. When you consider it recently reached 10 million players, it is hardly past its prime, and is still worth creating more DLC, or working on a sequel thats bigger(expensive) than the previous titles. If those 10 million players purchase vermintide 3, or new dlc, you are talking about a lot of money, and a /flex worth level of success.
So dont fret over Darktide, and a lot of tiders not liking it, that was the point, why compete with itself? Instead rejoice in knowing that if darktide succeeds it means bigger better things for Vermintides future, since both games will be tied to Fatsharks growth.
Also if they make enough money, they can buy back there independence, and finance there own games, rather than have to rely on Tencent. So its a win/win for everyone.
If anything with all the current backlash from Vermintiders towards Darktide, it more likely GW considers revoking the license, despite Vermintides success, and we all end up missing out because people refuse to accept Darktide is not a vermintide sequel.
Imagine them making so much money off darktide, that they are able to throw money from it, at a new vermintide (on top of what its made), with all the previous vermintide maps ported to it, and the addition of a new campaign, with as many classes as you want, because moneys rollin in from every side.