I can see the analogy of being indie musician and this - total income might not come from your main work but from all kinds of sources that is somehow related to it. In this merchandise.
https://www.emso.de/unheilig_2
I sometimes feel a bit cynical about it, but I'm glad they found a way to fund their tours & keep churning out more of my favorite albums every year.
Edit: To be closer to topic, Panic [1] used to sell t-shirts & hoodies, Atebits [2] used to sell pillows of the Tweetie app icon, and The Iconfactory [3] sold models of the Twitterrific Ollie blue bird.
[1] https://panic.com/blog/panic-goods-cheaper-shipping-coda-shi...
[2] https://technabob.com/blog/2010/03/03/tweetie-pillows/
[3] http://www.cardboardspaceshiptoys.com/toys-c-3/ollie-the-twi...
I would argue that now is a better time than ever to be a musician given that with the internet, a record label is not strictly necessary and you can market and sell merch to a massive audience and make money with just an internet connection.
In the last 10-15 years maybe, but up until the nineties or so, merch (even t-shirts) wasn't much a thing for most indie musicians -- except in the metal world perhaps.
As for tours, they were seen as loss-leaders for album sales. Some select concerts in big cities could get money, but the expenses of touring around e.g. the US where big too, and in the end it was a bet in getting even.
On the other hand, in the vinyl era, indie musicians (which had better profit-sharing deals and recorded cheaply, not spending $200,000 for farting around in the studio for 6 months) could sell 30.000 - 50.000 albums or so and be able pay the rent from that.