You can't frame the argument as a discussion of essentially swapping Holmes for McFadden, and then decide midstream to switch it to a comparison to Ben's case and a claim of hypocrisy. I mean, you can, but it's really two separate arguments.
Fact is, McFadden is playing better than Gay did at CB last year as a starter, and he's playing better back in LeBeau's system than he did in Arizona. As bad as the CBs looked at times against Baltimore, fact is we've given up 2 passing TDs in 4 games, only one of which was meaningful. Over a 16 game season, I'd take that any damned day of the week.
On McFadden v. Holmes: which is more likely to be suspended for a year? Holmes, no doubt whatsoever. McFadden has ZERO pot or PED dings. End of comparison there.
Fungibility: If you want to upgrade or replace a starting WR or CB, how easy is that to do? Assuming the Steelers FO methodology of free agency, we're looking at either bringing in a guy like McFadden cheap, or going to the draft. Good CBs who can play in their first year or two generally are R1 or R2 guys. We've taken some day 2 guys, with mixed results. Haven't had a high level CB since Ricardo Colclough, who was a fail; he's now playing with Jeff Garcia and other luminaries in the UFL, which appears to be the level to which his talent rises. Given Ike's age and uncertain depth at CB, we might want to draft a corner early in the next couple of drafts, but McFadden appears at this point to be an acceptable replacement. That switch, of Holmes for McFadden, then opened a hole at starting WR. We took Wallace in R3 last year, and IMO Wallace is an adequate replacement for Holmes; he'll need to work with Ben on the underneath routes, but clearly has the best deep speed we have ever seen wearing B&G. In sum: you could replace Holmes with a R3, but likely would need a R1 or R2 CB to replace McFadden (there are late round CBs starting and playing well, but recent history is against the Steelers being one to do so going forward: McFadden was a R2, IIRC). So from that view, we did better to reclaim McFadden than to retain Holmes.
On the comparison of Holmes v. Roethlisberger, the "hypocrisy" of keeping the $100 million franchise QB over keeping a WR in his last contract year who is in no way comparable in terms of team impact...? I don't know that this was ever a moral consideration from the FO, although the press widely inferred as much. It was more an issue of contingencies and replaceability. Holmes's replaceability, discussed above, was an adequate address of needs. You can look at it this way: we needed to upgrade starting CB, and get a long-term solution at WR2 since Holmes was far too unpredictable and likely far too expensive for an extension. We solved both emergent problems by dealing Holmes and drafting Wallace, or a net cost to us of a R3 rookie draft pick and contract. Sweet.
For Ben, if he is stupid enough to put himself in a bad situation again (in the path of potential liability), yes, he could face a year suspension. But again, you have $100M invested; the FO consideration is what is best for the team, not some mythic moral Steeler ideal, however much we fans invest in that; finally, what are the likely outcomes if you deal Ben?
First of all, a team would take a chance on Ben as QB, despite his perceived baggage. The real issue would be, once risk is assessed (reward is pretty well established as a top-5 QB), at what cost? Assuming Ben's contract is no small cap consideration, and when talking what you can get for a player, it's always roughly UPSIDE - (CONTRACT COSTS + RISKS) = DRAFT PICKS.
From there, you'd need to know, from a Steelers perspective, that the draft pick would include a R1, end of discussion. The best QBs of the past decade were all drafted R1 (or with Brees, what would now be included in R1) save of course the Wunderkid Brady. Manning was 1.1 (1998), Ben was 1.11 (2004), Rivers was 1.4 (2004), Aaron Rodgers was 1.24 (2005); emergent elite potential includes Matt Ryan, 1.3 (2008) and Sam Bradford, 1.1 (2010). Romo was undrafted (2003) but neither is he elite in my estimation. Warner was undrafted, but you're all the way back to the 1994 draft.
For all that, you're going to see R1 is historically littered with QB flops as well, so you either need divine intervention (Brady, Warner); a first round pick and long odds (late R1s such as Brees and Rodgers); or more likely, a top-10 pick and no small measure of luck as well (the rest).
Not every year yields an elite QB, so there's timing: just because you NEED a franchise QB, doesn't mean the Kiper hype du jour sitting at the top of the heap will measure up. Stafford? Tebow? Sanchez? Freeman? JaMarcus Russell, laughably not. Vince Young, no. Alex Smith, no. Eli, no. Flacco, no. Quinn, another laugher. Cutler, no. Leinart, God no. Losman... who?! Leftwich, anyone? David Carr??? Grossman??? Boller??? How about Harrington, or Patrick Ramsey?
So you are better off picking top 10, which limits trading partners by a third, and you need to hit a year when the players equal the hype, maybe once every 3 years. If you don't mind the odds of getting a non-elite R1 QB about 75% of the time, if good enough is good enough, or even outright fugly is a risk worth taking, then fine, trade away your $100M QB because of some perceived hypocrisy. Be willing, of course, to skip the Super Bowl for the next decade or so, unless you somehow luck into a guy like a Bradford that you can take a few years to develop.
To me, the whole thing is clearly a case of working with your QB to help him get his personal life together, because (as Steelers history post-Bradshaw has shown), it's no easy thing finding a QB who can finish the puzzle for you. The number of teams who win with a Trent Dilfer are the clear minority in the modern game.
You would probably need a team to surrender a top-10 R1 pick for three successive years to have a shot -- just a shot -- at getting talent at or close to Ben's. Meanwhile, Holmes + R3 draft pick = McFadden + Wallace. I'm totally fine with that. You can re-imagine it as: is Wallace a worthy successor to Holmes? And is McFadden playing CB as well as a R3 pick would have? I say yes, and yes.