Jax's cap number for Dareus going forward isn't that problematic despite the gaudy-looking numbers - https://www.bigcatcountry.com/2017/10/30/16569484/marcell-dareus-trade-2018-nfl-salary-cap-jaguars. Bear in mind that if they want to keep him, they can convert some of the future salary into a bonus that can be prorated.
Anyway, the problem he presents is as follows: other teams want the elite talent that he can be, but they're cautious not so much because of the money, but because that elite talent is one toke away from a full-season suspension. People need to bear in mind that Jacksonville acquired him because they want to win this year and think they can, and not because they're trying to perfect their salary cap situation. If you think a player represents an elite talent who can be a difference maker, then you pay the going price and don't look back. I'm not saying he's going to pan out, but in the two seasons he played for Marrone, he was dominating. Anyway, the cap issue likely wasn't a huge problem for them (they were $33 million under the cap anyway).