First I'm answering the question on the tittle:
In basic terms for an indie developer financial success mean first that the time invest, and assets purchased to make the game has paid off. (Usually the most expensive thing is time, and wathever the dev pirate, buy or make himself the assets, that's also inversion of time). In second had, finnacial success means that you win enough to make at least a second game.
Let's say someone works full time in the game (8-10 hours a day like myself) and having in consideration that he respect his own time and it's not thinking to eat from the trashcan in even days. That's between 80 and 200$ a day depending on country where he lives (ask for senior programmer/graphic designer salaries). Let's say 150$. So if you work on a game 25 days a month, you have to make 3.750$ monthly to consider it a success. (It, as I said, depens greatly on where you live in and what are your economical neccessites).
Let's say you are on Patreon; It's not like you have to make that money the first month. But the day you make more than 3.750$ monthly, that surplus is only paying for the months that you earned less. It will not be economically successfull until you are paid for all the time invested making the game, when you were earning 20 bucks a month.
Patreon and ungoing games with an unclear delimited end game from design is complicated to calculate when a game had been paid off/ consider it finnacially sucessfull. Furthermore, patreon job may include sharing art that is not technically into the game but uses hours from the artist.
So let's talk about a game with an end that you publishes on Steam, per example.
the game cost you 18 months and you are happy to think that your effort is valued in 2.000$ a month. You made all assets or (god save your soul!) pirated them. That's a game with a production cost of 36.000$. You put in on sale, first month you get 5k, second month 2k, thirs month 300$... But on the long tail and with sales, one day hit the 36k mark. Then the game has just paid itself. As an indie developer you can consider it sucessfull; And from that day on everything is profit;
But if you wanna support yourself and have time to make more games, you need to earn more than just paid the time you invested in.
So... That's answer it. Finnacial success depends partially on what the developer need.
About how important Marketing is, it's the most important thing, but sadly, the most expensive.
A bad game will big marketing will sell in Steam (or get noticed in patreon and the 0.percentage that pledge) big time more than a good game with low marketing.
Rape day is possible to get x100 more supporters than any very good game in patreon. Why? Because Rape Day is marketed now to thousands of people that wanna play NSFW games and don't even now that patreon exists, or that Patreon allows Adult game to be developed. Is even possible that 70% of players on Steam don't even know Adults games are allowed on Steam. Maybe know after rape day, 5% more of Steam people knows there are uncensored games on Steam.
But not talking about extreme cases of free marketing; It is imperative to make the most marketing one can do with no budget (trailers, going to all forums, having a webpage, etc, because anyone how can dump 5k on any kind of paid publicity will have x1000 more reach than other that cannot put that 5k.
