Learn Turkish: Aksine, günümüzde asıl önemli olan şey sadelik. - On the contrary, nowadays the thing that is truly important is simplicity.

Yes, you can. When used at the beginning of a sentence to mean "on the contrary," aksine and tersine are interchangeable. They both signal that the new statement completely contradicts what was just said.

Why do we say önemli olan şey instead of just önemli şey?

While önemli şey simply means "an important thing," adding the participle olan (from olmak, to be) creates a relative clause meaning "the thing that is important." This highlights simplicity as the specific main thing that matters, matching the English translation.

What exactly does asıl add to the meaning here?

Asıl means "real," "actual," or "main." Placed right before önemli olan şey, it translates to "the truly important thing" or "the main thing that matters." It strongly emphasizes the contrast set up by aksine at the start of the sentence.

How is the word günümüzde built to mean "nowadays"?

It is built from the root gün (day). First, the possessive suffix -ümüz is added to make günümüz (our day). Then, the locative suffix -de (in/at) is attached, literally meaning "in our day," which translates perfectly to "nowadays."