Maia hasn't been a guard guy since 2009; did he even pull guard against Anderson, and haven't his guard-pulls been few and far between since then with very little happening from them? And his shining achievement off his back is submitting Chael Sonnen.
Maia's not some guy like Miguel Torres or Rodrigo Nogueira who relishes in fighting off his back and every single person who's been there has had to deal with a lot of problems. He avoids being there except when all else fails. Even Shields beat Maia by taking him down and holding him in guard because he knew he's gotten way weaker there since re-distributing his focus to his shoots (and his guard wasn't the strong part of his grappling game anyways; I don't know where Joe Rogan got the idea that Maia's one of the best guard-players in the sport). Shields lost every round he didn't do that, and didn't Usman do the same thing?
Shields' Jiu-Jitsu style doesn't really offer much in terms of dissimilarity to Askren. If anything, if Shields' top-game style was all it took to overcome Maia's guard, it helps out Askren's case.
When people say things like "Maia outgrappled Fitch, omg", they're implying a lot more than taking him down and riding his back for 15 minutes.
Did people expect Fitch to escape Maia's back-control? Why?
You can't just use vague buzz phrases like "avoiding a sub" or "superior submission grappling" when saying how a fight's gonna turn out (even saying "Askren would win a UFC title because of his wrestling" isn't speaking clearly). How, exactly is it gonna happen? Because based on Maia's fight history and how he performs against opponents who have skillsets similar to Askren, it's not likely (at all) to be because of his guard-work. And assuming it would work because "Askren's Jiu-Jitsu's sloppy, therefore Maia will just lock a triangle on him like Chael" really shows a lack of knowledge of grappling; there've been enough videos talking about Askren's grappling skills for me not to explain why he's far from sloppy. Askren doesn't spend time sitting in conventional guard positions, so how is Maia gonna set up one of his textbook moves (because, as we all know, Maia's known for his textbook Jiu-Jitsu)? Karl Amoussou showed more danger off his back by submitting Bryan Baker with a heel-hook than Maia has in almost 10 years, and Askren shucked off everything he threw at him
This is the biggest thing: what makes people think that Askren doesn't have the balls to go to the mat with Maia? Nothing about any of Askren's fights would suggest that, so bringing up a hypothetical kickboxing match with him is like talking about Gaethje engaging in a cerebral grappling duel.
If Maia would beat Askren, it'd be the same way he beat Fitch or any of his other wins over the last few years: takes them down, advances to their back at some point, either locks in a rear-choke or rides his way to a decision. And, based on his hit-or-miss takedown history, that's not a safe bet against someone like Askren.
That changes the fight into a matter of who's gonna get the takedown first; neither fighter's path to victory lies on their backs, so establishing top-control and implementing your game from the beginning is paramount. If it's a close fight, it's because they spend rounds alternating that role. And based on Askren's wrestling background [I think he's ranked as one of the 10 best collegiate wrestlers ever] and his "I shoot for a takedown 3 seconds into a fight, and I usually get it because I'm a better wrestler than everyone at welterweight" history to start fights, it's safe to say he's got a better chance of pulling it off than Maia does. Their comparative martial arts backgrounds also supports the idea that Ben could stop Maia from taking him down than Maia could stop Askren.