During the enfeebled showdown that caps Peter Segal’s “grumpy old men come to blows” dramedy, television announcers remind us that old rivals Henry “Razor” Sharp (Sylvester Stallone) and Billy “The Kid” McDonnen (Robert De Niro) both walked away from boxing while at the height of their powers and with virtually unblemished records. Having endured almost two insufferable hours of punishingly lame gags and limp training montages (rest assured, raw eggs are as disgusting as ever), viewers can’t help but be taunted by a “what if?” scenario in which these screen icons demonstrate a similar desire to go out on top. With Stallone moping nobly and De Niro mugging shamelessly, their demeanors rarely suggest that 30 years of resentment are reaching a simmer, much less a boil. The other relationships are similarly unconvincing, with Stallone’s romance with Kim Basinger (seemingly in an opioid haze) proving spectacularly anemic. In turn, the seniors’ climactic bout might be the most dispiriting scene to unfold in a boxing ring since a broken-down Mike Tyson surrendered to Kevin McBride back in 2005. At least Tyson had the decency to admit it was just a cash grab.