I took the liberty of looking at the Total War channel and discovered that Troy will be free at launch if you claim it in 24 hours.
CA aren't exactly gonna put out a entire game for free for it is likely that Epic Games is paying them a significant amount of money to develop the game for them or even is straight up covering the development costs. This isn't something new to the industry as it happened with Bayonetta 2, which only exists because Nintendo paid Platinum Games for the development.
This means that the only reason Troy exists in the first place could be funding form Epic Games. Saga games are essentially independent projects aswell so this means that it only applies to the one title. Don't forget that Thrones of Britannia sold poorly overall so if Epic Games offered CA a guaranteed return on their investment on top of letting them sell it on other platforms a year later, its a dam good deal and gives us all a game that would not exist otherwise.
Warhammer however is not a standalone titles like the Saga series, it already has 2 games and a ton of DLC available on steam and CA aren't stupid. Remember this is form a company that hid whispers in a trailer 3 months before a DLC, that none of us picked up on, they aren't stupid, they are more clever than people are giving them credit for. They will make Warhammer 3 available on Steam. The one realistic possiblity is that they might make it available on both
"Have A Little Faith"