Tinker Tinker I agree with you, but I also want to acknowledge the significant role that SL clubs play in solving the long-irons for many people.
While I agree that the greatest problem with long-irons is probably lofts, club length is a big consideration -- we hit shorter length irons in the centre of the club more consistently and control the club face better. These considerations are significant for achieving the ball speeds and launch angles needed to get these irons to perform consistently and to preserve yardage gaps. The shorter, SL clubs, I believe, are really effective for dealing with this challenge.
At the same time, like you say, this does not solve the greater issue of generating enough SS to get the lower lofts to spin the ball and optimize carry. In fact, the shorter shaft lengths even magnified this issue. Most people have difficulty launching irons that are lower than 25* and getting them to spin the ball enough to optimize carry (I believe Tom Wishon even says 27 degrees). That difficulty increases as swing speed decreases. When I was younger, the 4-iron had a loft of 24 degrees, so it was an iron "on the bubble." If you had the swing speed, you could hit it pretty well, but it was more "iffy" than the 27-degree 5-iron. If you didn't have the SS, it became inconsistent and a problem club. The 21-degree 3-iron (and even worse, the 18 degree 2-irons) were trouble for almost everyone.
As you say, now, in the era of 20/21 degree 4-irons, these have become problem clubs for most people and they are the reason that hybrids and FW come into play for many/most people at this loft. Many people are trying to hit a 4-iron when they would be better off hitting one of these alternative clubs, or just hitting the 5-iron into a good position, rather than hitting 80% of their 4-irons into some kind of problem. (Rich Hunt and Mark Broadie's work in regard to the stats of scoring have interesting implications in this respect.) This is the reason I made the earlier comment that a 175-yard carry with a 4-iron probably indicates a slower swing speed than is necessary to use this club consistently well.
Sorry for the hijacked thread.