The audience deserves sincerity
Communication is writing's basic challenge. Can you, the writer, communicate your thoughts to an audience? It may be a sonnet, meant for an audience of one. It may be a garbage disposal installation manual, meant for an audience of twenty-thousand.
It may be a blog post addressing simplicity and sincerity in writing, meant to detail a writer compensating a reader for their time.
Straightforward, honest writing is a craft you must learn.
Trust yourself to trust your audience
The writer owes the reader sincerity and confident communication. Simplicity is the written reassurance that in this transaction of communicated knowledge, the reader needn't fear. The reader won't be tricked. The writer will not talk down to the reader. The writing's intent will not be opaque. Only trustworthy writing deserves to be read. Only confident writing earns its audience.
Simple writing can achieve all the flowery flashiness found when a writer is rushed and can't edit their mocha-flavored crutch-words out of the document in time.
Sincerity is only vulnerability when there is no trust. A vulnerable writer may not trust their audience because the writer does not trust his or her own authority. This self-distrust from a writer often reveals a cynical mood. Cynical writers will protect themselves. They disrespect the reader. They hide in insincerity. This destructive act kills the writer's craft, weakens the agreement with the reader, and reduces the audience size.
The answer is simplicity. Simplicity does not destroy the writer's voice. Sincerity does not mean forgetting the fun of language. Connect with your audience. Communicate to your reader. Trust your ability.