The fact that most game developers nowadays support the game so much and especially so long really makes some think that that is standard, but really, it is very expensive to support a game with more content for even a year.
It's not expensive for two people to stop masturbating and playing video games long enough to work on content for an indie game they developed from their parents' basement on a $200,000 budget. They're self-employed, cost isn't even really an issue.
And no, supporting your game after release IS the industry standard now. Pretty much every game released in the last 5 years has had some kind of downloadable or post-release content. Not supporting your game after release is substandard service, especially when your game ships with so little initial content.
Aside of that, will they be able to get more sales from adding more content?
Yes.
Yeah, FTL was really lacking multiplayer and was so barebones and devoid of content. Oh wait, no it wasn't. multiplayer wouldn't really work (isn't like half of the experience the feeling of isolation?), and it was already full of content.
I'd say 8 levels is pretty barebones for a roguelike. Hell I've played freeware roguelikes with more levels and more level diversity.
And not having multiplayer is a big deal. Multiplayer in FTL could totally work, the developers are just lazy forgets that don't feel like implementing it.
They need to get paid and they needed money to get the game out there. I like that they didn't go adding pointless features as stretch goals, they chose a realistic stopping point and actually shipped the game. That's more than can be said for others.
The problem is they shipped the game and just forgot about it. It's substandard, deplorable service especially when there's no good reason that the developers can't support their game and do something for its fans. Again, look at Terraria. They didn't have any of the budget or experience that the FTL developers had, but they released the game on time and continued to support it through huge content updates for free.
The FTL developers are too loving lazy to even release paid DLC to the point where their game is almost entirely irrelevant these days.