Do you believe that you can do a better job than those tens of guys who contributed to a DBMS? Go ahead.
Do you not need reliability, integrity, security, performance? Go ahead.
So use a file to store data instead of database but know the trade-offs.
As soon as you start processing forms and doing anything transactional based on that, you'll be quickly reinventing a lot of wheels if you aren't using a DBMS of some sort.
Doesn't Hacker News run on flat files?
And if you e.g. have multiple dataflow paths with different priorities then you also probably shouldn't be using a DBMS, because they're too much of a closed world. Using database technology as a library would be a better approach; otherwise sooner or later you'll find you need to customize the internals of your DBMS, and with most extant products that's somewhere between hard and impossible.
Could you expand? Does this mean you wouldn't use an ACID database for CRUD?
99% of the time all you want to do is a) commit input events to a durable store as quickly as possible b) read a state-of-the-world that's consistent in the sense that it represents all the consequences of all the inputs up until some (logical) time t, and IME an event sourcing model is if anything better at doing that than an RDBMS is.