Send "soft power off" signal from a Mac via HDMI to a TV
How can I make a TV (a Sony W755C in my case) do a &quot;soft power off&quot;
(same as when pressing the ON/OFF on the remote) by sending HDMI
signals from a connected Mac?

You can't.
According to Apple:

Do Mac computers with HDMI support CEC (Consumer Electronics Control)?
Mac computers don't support (and can't control) other devices via CEC
connections and HDMI.

Basically, the controls are simply not there to send to your TV.  To do so, you need the USB adapter.