Volkanovski has been taken down quite a few times in his UFC career, as has been covered, always by lesser and smaller wrestlers than Islam. To his credit, his scrambles and get-ups seem to be top-notch and he won all of those fights. But I don't know if you can rely on being short, stocky, scrappy and having good scrambles against a guy like Islam who's so good at chain-wrestling, throwing out a variety of takedowns in rapid succession, and consolidating position by locking down his opponent to set up shop on top once he drags them down. I agree that Volkanovski -- in a pure skill-for-skill, pound-for-pound basis -- is one of Islam's most tremendous tests to date, but fundamentally Makhachev isn't really being deprived of any of his weapons and doesn't have to change his gameplan overly much. Conversely, Makhachev is the biggest, most elite grappler Volk has ever faced by a large margin and is very defensively sound on the feet. He will likely be able to negate Alex's own offensive wrestling and (to a slightly lesser extent) clinch games -- two things Volk relies on pretty heavily in many of his outings -- or at the very least make them incredibly risky to employ.
What also worries me is that Alex isn't making any concerted effort at bulking up for this fight at all and has announced his intention to basically not cut weight and instead just retain as much lean muscle as his frame naturally carries as it is so he can come in a bit heavier than he normally would for a Featherweight showing. Alex is a fairly strong, muscular guy for the weight class but he's not the biggest 145er we've ever seen. Meanwhile he'll be facing a Lightweight who is by any metric tremendously strong and big even by the standards of that weight class. It's sketchy. I know, Alex feels he fights better at his current frame and it serves to preserve his cardio, pace, footwork, and speed but... I don't think it will do him any favors once Islam gets his hands on him.
That being said, I still can't bring myself to say that Alex has no chance in this fight. I do think he is a live albeit remote underdog. His fight IQ, cardio, composure, footwork, game-planning, and ability to make mid-fight adjustments under fire are not to be underestimated. I genuinely believe that Volk is a generational talent and a tough out for anyone two weight classes in any direction. I could see a reality -- somewhere out there -- where Alex frustrates Islam with his footwork, complicates his takedown entries while peppering him with boxing combinations and low kicks, continuously scrambles out of bad positions, survives some bad spots on the ground, maybe lands some solid shots that manage to hurt Makhachev, and ultimately ekes out a closely-contested win by Decision or perhaps even late TKO. I think it's within the realm of possibility for a fighter as skilled as Alex has proven to be... but I just don't see it as the most likely scenario, all things considered.
I think it's far more likely that Islam makes his reads early, Alex throws out some feelers, maybe lands a couple of shots and escapes the first few takedown attempts, but eventually Islam finds his hips and really drags him down and keeps him there to ride out the clock even as Alex tries to scramble like a madman to no avail -- all as Makhachev rides the bucking bronco. Rinse and repeat until the clock runs out to an Islam UD, 49-46. One round in there for Volk where his scrambles are on-point and he lands some clean shots, I guess.
I think it's feasible for Islam to finish Alex, but it would be a late TKO or Submission I imagine. Rather than Islam catching Alex on the feet or snatching up something crazy in transition, I think it would just be Makhachev slowly consolidating position and grinding Volkanovski into dust during the championship rounds with GnP or an arm-triangle/RNC/armlock after trapping him in a bad position following a desperate failed scramble where he only got pulled deeper in like quicksand. All in all I'd probably take Islam by Decision, though.
A disclaimer, however. If things had worked out differently due to cosmic shenanigans and we had been able to see Volkanovski spend the last year or two defending his belt against the likes of Movsar Evloev and Bryce Mitchell in a convincing fashion (i.e. repeatedly stuffing the vast majority of their takedowns, scrambling out of bad positions, punishing them on the feet, and winning a dominant Decision or even finishing them)... and if Volk was willing to properly spend time packing as much extra lean muscle as possible onto his frame in order to properly move up to 155 pounds for this challenge alongside a lengthy, dedicated defensive grappling camp... I would view this as close to a 50/50 fight. Hell, I might even pick him in that case as it would be far easier for me to visualize a proven route to victory for Alex at which point it would be essentially a matter of "scaling it up" against a larger and slightly more talented/well-rounded fighter in Islam (when compared to the aforementioned Evloev/Mitchell).
As it stands, however, the way they match up stylistically I feel like Volk will be inherently deprived of too many of his own weapons in this bout and picking him would require me to take a lot of things on faith while hedging my bets entirely on his observed intangibles (elite intangibles though they may be) and his performances against vastly inferior and smaller grapplers. I am not comfortable doing that, personally.