"A thinner enclosure isn't entirely out of the question if Apple cuts features to reduce costs, but at the same time, reducing size leads to lower battery life, and Apple probably wants to have decent battery to compete with or outmatch cheap Windows laptops.
Since the MacBook Air can run fine with an M-series chip and no fan in an enclosure that's 0.44 inches thick, there's no real reason for the MacBook to be any thicker than that, unless Apple wants to add a bigger battery."
Max thickness of the 12" MacBook is .52" with average thickness .30. So I'd be OK with .44" but that is crazy overkill. If they just took out the Intel processor from the 12" MacBook and replaced it with A18, it would likely get over 15 hours of battery life. What the heck are they going to do with all the extra volume?
On top of that, to fit a 13" or so screen, the length and width will also have to grow (no that much) but adding even more volume.
But make it any thickness Apple wants, but please get the weight down to around 2 pounds like the 12" MacBook. When the M1 Air came out, I bought one, but returned it the next day. Going up to 2.7 pounds may not sound like much, but picking up and moving my laptop maybe dozens of times per day at home and out turns out to be a huge lifestyle change. Even with a slightly larger screen, it should be an easy design problem to keep it down to 2 pounds or even less as you can have a smaller battery and still get world beating battery life versus anything else approaching that weight (other than an iPad).