True, they will just have Disadvantage on attacks and saves.
And that's right now, once we get the ability to take feats, it`s possible they can burn a few feats and don the armor, and spend the whole day fighting with no penalty. They just can´t cast spell.
No sane player will do this, probably, but the character have the ability to do so. So whatever is the thing that impede spellcasting in armor, is not that "Most wizards can't wear armor in combat because they aren't strong enough to wear it all day". Once he burn the feat, a wizard with str 8 and plate armor can wear the armor just as easy as a cleric with str 8 and plate armor. No more, no less.