Mage Armor with a decent Dex (16) is AC 16 which is the equivalent of chain mail. It lasts 8 hours and doesn't require concentration. Its one of those non-choice spells that is so good you always have it prepared or on a scroll.
I asked if you found this true in play, and you responded with theory again - theory based on a specific ability score and spell choice despite the small number of choices early on, and the inability to know how long a day will be in a given scenario (it lasts 8 hours, but the day is 24 hours, and monsters don't just wait for you to cast it again). So I guess that answers the question - you have not, in fact, found it to be the case in actual play, and every conclusion you've drawn is based on faulty hypothesis and averages which don't account for anything varying from the center of that average.
Since the open play test documents state that an average day is 4 equal level encounters,
And that's more theory based on averages rather than what actually happens. I think I have my answer.
You should maybe try the game you're critiquing. I think you will find it plays different than you might think.
TL.DR Your Wizard was doing it wrong.
Complete with "badwrongfun" for a game you have not even played, in response to someone actually playing it?
Yeah, the only badwrongfun is not even playing. That, that's badwrongfun.
You now have multiple reports from people who have actually played it, and found their experiences differ from your hypothesis. Results trump hypothesis every time - that's how science work, and it's why you make the hypothesis to begin with, to test it in practice and see if it holds up. So far, it has not.
Four encounters is an average not a pre-programmed fixed sum in a zone. So half your days will have more than that (sometimes a lot more), and half less (sometimes a lot less). 8 hours is only 1/3 of the day, and things don't just conveniently wait for you in the chaos of the adventure. Spell slots are often used based on immediate survival needs and circumstances, not average utility. And in practice, the mage get fairly easily get trounced if he's not extra careful, sometimes losing entire turns just to maneuver away from foes rather than casting a spell.
The game is a heck of a lot less predictable and static than some other versions of D&D. It's not based on set piece encounters and combats that are exactly on par with average party level and evenly spaced encounters and convenient resting places to recover. It's a much more dynamic style of play encouraged by these rules than some other versions encouraged. You should try it, rather than speculating about it. I think you will find it does not play like you think it should.