More demand? From who? Anthrax plateau'd with their fan base years ago, seeing them talk for three years about an album yet produce nothing is not creating demand, it's creating a joke.
And who says it's coming out next year? Scott said in an interview the other day he doesn't want anything out until they can tour. Touring is opening up in some parts of the world but it's to massively restricted audiences, or it's opening up and closing down soon after. Then there is Anthrax's reluctance to do anything more than be a third tier act on the festival circuit which will no doubt hamper them even more given that festivals with limitations wont be viable for promoters.
You can also add to that scenario that even if it's released by the end of 2021 the band are not going to spend the entire time actually writing and making something great, they will piss fart around, do sloppy youtube videos for other bands and friends and then spend three months writing something that is at best on a par with Kings.
Charlie and Scott ought to put their money where their mouths are and if they don't want a full length until they can tour, release an EP with two new songs and a few rare live tracks. If they are such good writers and these new songs really are thrashy, fast and aggressive then it wont be hard to write an extra few songs for the follow up album. At least an EP with something new would show the fans they care about the fact it's been 4 years since they produced anything new, at least it would show the fans they have something positive to offer them.