I wrote a big post of videogame economics from the perspective of someone who has been involved in the process of greenlighting projects and doing due diligence checks - not senior, but involved in the calculations and judgements.
I wrote... a lot for this post. And in the end, I'm binning that draft because it deals too much in the numbers. Lots of % and ratio bullshit to try to get across how hard it is to work, because that's what my job entailed. The key takeaway is and always was simple - DLC won't make a game profitable, it won't fix having a small audience, and unless you want to exploit people with poor impulse control you can't fix a bad pitch with "then we'll have in-game purchases."
Agreed with a lot of this, and very important stuff to remember as a dev.
Paid DLC is something to be deployed in special circumstances, where it makes perfect sense for your game and your audience. For every new idea you have, the options are either to "Add it as a free content patch", "Make it a paid DLC" or "Save it for a future sequel" and it's worth really carefully weighing those options.
I don't know what the average is these days but remember that for most paid indie games, only between 1% and 8% of the audience will ever buy a paid DLC.
In most scenarios these days, "Add it as a free content patch" is probably the way to go for most of the ideas and improvements you come up with. If you add stuff as a paid DLC, a large majority of your players are never gonna see it or touch it. So if you're doing that, you need to be okay with that (and okay with the fact that it's probably not gonna magically make your game financially sustainable either.)
Or if your game projects are short (and not "replayable"), probably just make a sequel or new game.