Hulk was a MCU movie with an A star cast and it bombed. DCEU will be 5/5 for 100 plus opening weekend movies (probably 6/6 after Aquaman) so I wouldn't put it past a Deathstroke movie making decent profit, especually if the movie has a lower budget. Cant say these things until the marketing starts.
Not really a good comparison, seeing as how Hulk came out right at the start, and Ant-Man came out in a post Avengers world where you could literally say to someone "it's connected to Avengers", and they would go and see it when they wouldn't have otherwise.
It might make a profit, but that's not the point. He's not going to get the buzz he could get until WB properly establish the universe.
"With the first link, the chain is forged. The first speech censured... the first thought forbidden... the first freedom denied – chains us all irrevocably."
You're talking about establishing a shared universe, right? Deathstroke doesn't necessarily need to be apart of that shared universe. It can be a movie all on its own. And besides, it won't be coming out in the near future. Who says the universe won't be established by then?