I don't think the case is easily made, Matt, but I'll tell you something that many might find surprising. Irving considered himself to be on a par with Mosconi on he ten foot tables, while he was quick to admit that Mosconi was a cut above him on the nine footers.Stu, could one not be justified in placing Irving shoulder to shoulder with Mosconi? Did he not win significant titles over a longer era in tougher competition? Weren't many of Willie's "world titles" essentially a single match against an annual challenger? Not to discount Willie's accomplishments, but it seems Irving really stood the test of time, and was a true pool diehard, competing and exceeding in multiple disciplines. I've heard him described as almost flawless at running out 9 ball in his prime by Danny D. What slowed him down was his break. I'm biased in these thoughts though, simply because I like his style overall, dress and appearance, great cues (two balabushkas), and the dignified way in which he carried himself. a True legend!
Still - if I have to watch someone play straight pool for an extended period of time it would always be Mike Sigel.
Edited to add: I know an old school guy who hung out at Beanie's and is not the type to be very complimentary of anyone, however, he has referenced when Hubbart brought around Sigel way back in the day. He said the routine was, if Sigel couldn't beat them, then Hubbart would beat them (keep in mind, Sigel was a KID and a champion). He said that Hubbart was very a very intimidating gambler. And this guy is not squeamish about that type of thing.
In the first forty years of the straight pool era (approx 1920-60), Irving was no more than the best of the rest, not far above guys like Caras and Lassiter at straight pool, but Mosconi and Greenleaf were a cut above everybody, and were almost unbeatable in the big spots, except by each other. As you note, Irving stood the test of time and was probably a top 10 in the world straight pooler until he was about 68 years old, but he didn't quite run the balls with the consistency of Mosconi or Greenleaf.
Charlie Ursitti, Mosconi's business manager, has taken note that of all those who have ever played straight pool, Mosconi was the most likely to win a game on the first inning. Eerily, though, Charlie added that Greenleaf was statistically more likely than Mosocni to win a match in two innings or less. Yes, their skills were that close, and nobody else could run as many as they did with similar consistency.
As you suggest, Irving was an almost flawless technician at the table and was the Efren Reyes of straight pool, meaning his defensive imagination brought defense to a level previously unseen. Irving could steal a couple of extra good looks at the table with his stellar defensive skills, but if wasn't quite enough to fill the gap between him and the top two.
On a final note, raising the longevity issue between Mosconi and Crane is just a bit unfair. Willie suffered a stroke in 1956, two years before I was born, and some say that, despite a couple of scattered successes afterward, he was never the same player again. Willie's retirement from competition at about 52 years old was, at least in part, a consequence of the 1956 stroke.
It has always been very difficult to compare players across the generations, but my pecking order for the top five straight pool players of all time is 1A Ralph Greenleaf, 1B Willie Mosconi, 3A Mike Sigel, 3B Steve Mizerak, 5 Irving Crane.
Guys like Nick Varner and Thorsten Hohmann can possibly be placed in the top five and they won't look out of place, but, despite being all-time greats, they just barely missed my cut.
Speaking on a personal note, of all the players I've ever met, Irving was the one that most caused me to fall in love with pool. I liked his appearance, his demeanor, and the way he conducted himself. When you chatted with him, you felt as if you were talking to the dean of an Ivy League university. His respect for the game would rub off on you.