I'm late to the party, but after reading through the thread thought I'd make a couple of points.
It is worth noting that being an enforcer takes a lot out of a guy, and that Wensink and Fotiu both probably did their best fighting before even reaching the NHL. By the time Wensink made the Bruins, he'd already been fighting his way through the minors since the early 70s...and was not nearly as fresh as he'd once been...hands get beaten up, muscles, joints get cranky...it cannot be easy to slug it out year after year.
O'Reilly was only two years older than Wensink, yet made the Bruins by the early 70s. If you watch his O'Reilly's) early fights, he throws punches like a jackhammer. By 1980, injuries began to catch up with him, and while he was still a very good fighter, he didn't have quite the same intensity. By 1977, when he made the NHL for good, Wensink may have already slowed down a bit....I have no video evidence for this, but he had a rep as a great fighter in the minors, as well as for being a wild man. It stands to reason that years of punching people takes its toll. Could be the same for Fotiu, judging from his rep from his WHA days.
Wensink didn't fight as much in the NHL as he did in the minors, nor as much as a few of his teammates. Yet stories of his minor league fights, as well as his insanity, created a mythological figure.
As for Cashman, his fighting really has to be separated into two stages....before his back injury in 1975 and after. He never skated as well after injuring his back, and his fighting was compromised pretty severely. He played the rest of his career in pain, and while he still fought well at times, he was nothing like the Cash of the old days. While he lost to Schultz in '74, I don't think he had an entirely fair shot....Dave kind of grabbed him and threw the first punch. Toward the end of the fight, it looked like he might get his left free, but they tumbled into the net and the fight was broken up. He played a great series though, he and Orr really carried a very lethargic Bruins team. He also beat Jim Watson
and Van Impe in the same series, though neither were particularly good fighters.
Most of Cashman's post 1975 fights were anticlimactic, the Kelly fight being a prime example. He had his moments, such as against Howatt, but for the most part his best days as a fighter were long gone.