IMHO, elusive targets will absolutely never be permanent content.
There is tons of available content within the WoA Trilogy for Hitman.
The other content is like candy canes, pumpkin spice lattes, McRib or Easter Cream Eggs.
If candy canes, McRib, pumpkin spice or Easter Cream Eggs were available all the time, then sales would drop overall. No one would eat candy canes or Easter Cream Eggs everyday all year.
Greater interest overall is garnered when content is seasonal or temporary like elusive targets.
It is one of the few means for IOI to keep a dwindling tail of sustained interest for players engaged in Hitman over the post-launch year.
From what I observe in terms of post-launch content so far, IOI have invested more effort in developing interest escalation contracts, and might not spend the same effort over the year in other post-launch content, such as elusive contracts or Special Assignments, or bonus DLC, Sniper Assassin content, and multiplayer Hitman (obviously, RIP at end of Hitman 2). If there was a lot of new post-launch content I would assume that IOI would have specified the post-launch content at launch when most were playing the game.
My guess is that there may be paid DLC content, in a year or two, like a Hitman v3.5, with an extended campaign more like the Patient Zero campaign, a self-contained story across many locations in Hitman2016, Hitman 2 and Hitman III, separate from the story arc of the WoA Trilogy.
It makes sense for there to be paid DLC content, given that it is pretty cheap for IOI to rejig a few locations and insert a few new targets. Enough players bought Hitman2016, Hitman 2, and Hitman III who would be in the market to buy a paid DLC. It is also better for IOI to wait a year or two to introduce new Hitman content as paid DLC, when those who had a year or two break from Hitman, would be more willing to come back for more.
Assassin Creed realized that yearly releases of their franchises was too much. Perhaps the best time to introduce a paid DLC is ~two years or so from now. Something halfway in the development period of Project 007 and their multiplayer IP or other, which is probably 4 years off or so.