Waqar's back injury undoubtedly didn't help but are you really telling me that you think if he'd not suffered it he'd have been able to keep bowling as he bowled between 1990/91 and 1994/95 until, say, 2000/01? I don't. I can't see anyone keeping-up such sensational brilliance for more than a few years. Frank Tyson between '54 and '57 was in the same boat. Also I don't think Waqar's pace was ever "dramatically" reduced as he was still bowling at close to 90mph against England in 2000/01 (despite very hot conditions) and 2001. At his very best, I can't imagine he was ever much more than 93-94mph at top speed (no-one ever talked of him being as fast as Shoaib Akhtar in his prime, whose average was 88-92 sort of range). I'm sure the injury took a bit of pace off him but I don't think it was much more than 3-4mph.
For Steyn and his 90mph outswinger read Waqar and his 90mph inswinger (though of course Waqar could also make the new ball swing out, while Steyn is nowhere near as skilled with reverse-swing as Waqar was). That fast inswinger which smashed so many stumps and boots was what made the Waqar of '90-'94 the most devastating hurricane the game has seen. Likewise, Steyn's fast outswinger is everything to him: if he can't bowl that outswinger he's anodyne and (inevitably) expensive; if he can (and so far this has been in the considerable majority) he's quite deadly, usually despite expensiveness rather than in addition to economy.
Neither Steyn nor Waqar were one-trick ponies, not at all, but they were\are both bowlers who lack the tall bowler's ability to tie a batsman down. So they are both best-served by attacking, attacking and attacking. Most short bowlers can't attack as well as those two could\can, because they aren't accurate enough with the really full length and aren't as good with the seam-position. However, neither of them are capable of bowling accurately enough to tie batsmen down over long spells and many consecutive games, because as a short bowler that requires almost unbelievable accuracy (Malcolm Marshall-esque). So therefore they should not try in my book.
Not being able to sustain the sort of form Steyn has shown the last 2-and-three-quarter years is not due to losing the script; it's just due to the fact that no-one can bowl so devastatingly for more than a few years at a time. It's just beyond capability. Steyn is bowling as well now as he is capable of in my book and no-one can bowl at the top of their game non-stop for 10-12 years.
RIP Fardin Qayyumi (AKA "cricket player"; "Bob"), 1/11/1990-15/4/2006