First, I'm pretty sure there's some kind of cap-relief for career-ending injuries. But, that's a fairly strange argument-against in the first place. Why sign ANY highly regarded player, if anyone could blow out a knee tomorrow and screw their team? That is what injury-settlement insurance is for.
17M in guaranteed money equates to a little under $3M a year in cap hit. The other 32M in salary is paid in a backloaded contract.. ESPECIALLY in a 7-year deal.. the player and team pretty much are of the understanding that they'll never see years 6/7 of the contract. The only thing it guarantees is that the player will be able to renegotiate and see another payday, and/or the team can release them without much damage done.
Thus, if the guy is making a base salary of $1M or so the first year, his cap number is somewhere in the ballpark of $4M... for a guy who is leaps and bounds ahead of Sims in terms of talent.
The only reason this deal doesn't make sense is if the Raiders are cash-flow poor and don't have the jack to pay upfront money. In terms of simple capology and talent.. keeping Sims at 6.5M and passing on Steinbach at $4-5M is absolutely moronic.