Despite being a 15 round fighter, Michael Spinks never lost at 175. I don't remember him ever being knocked down at that weight either (Anyone? If so, please remind me) despite beating Eddie Mustapha Muhammad, Dwight Muhammad Qawi and Marvin Johnson at that weight. Gerry Cooney was shot to bits, but was still a big puncher at heavy and couldn't knock Spinks down. Artur is a helluva puncher at 175 and a terrific boxer as well, but there is no indication that he is stopping Spinks.
Spinks beat Qawi, an elite skilled pressure fighter with great defense who, unlike the Beterbiev hypotheticals here, actually did win a title at cruiserweight and went 15 with Holyfield as an alcoholic. Not only that but Spinks had him wary of his power too - very unusual for Qawi and a testament to how hard Spinks actually hit. Spinks clearly imo. I wouldn't be surprised if he timed an awkward shot to seriously hurt or even KO Beterbiev.
Beterbiev didn't have problems winning rounds against a slick boxer like Bivol who was well schooled, had GLOBAL amateur experience and a long record so it's not impossible that he wins rounds against these 1980s demi-gods that people like to put on a pedestal around here. Spinks probably beats Beterbiev by a comfortable UD but it wouldn't surprise me if he walked into a trap the way Callum Johnson and Smith walked into while Beterbiev was going backwards and had his back to the ropes.