This is a very good point. Let's have a look. In exchange for +6 AC, proficiency with Con saves, +4 hit points, and a few minor perks:
That's a massive hand-waive you just did there.
First, it allows you to dump Dex as a stat. Two PCs who both have a 10 dex (though you could even go 8 dex now), and now it's Mage Armor 13 AC vs. Plate (+8) + Shield (+2) + Defense (+1) = 21 AC (so that is +8 AC, not +6). AND you had to choose the Mage Armor spell for your spell book, and spend a first level spell slot using it, and it's only lasting 8 hours from that casting. All that is something the mutli-class character doesn't need to do...and you mentioned the loss of a first level spell slot, but Mage Armor is that spell slot so that is a push.
Next, you have proficiency with not just that armor and shield, but also with all those weapons (and mounts).
Then you get Second Wind, which can save the life of a Wizard, particularly at low levels.
And you get more hit points, to go with that second wind.
And you get proficiency in both Strength and Con saves, not just Con saves. So now you have four of the six stats with proficiency.
And finally, you're now only one level of Fighter away from casting two spells in one round with Action Surge!
That's a heck of a lot more than what you listed.