That won’t help you in the unlikely event the Immigration Officer asks for one. Some people “rent” a real ticket out of Thailand from onwardticket.com or one of their competitors. I think they are about $15US and automatically cancelled after a couple of days.
Three weeks ago, when I landed at Phuket, the Immigration Officer asked me for my onward ticket. Fortunately, I had one. Had I not had one, she probably would have frowned on me leaving the line and booking one. I understand this is a rare occurrence, but it happened to me.