EzekielRaiden
Follower of the Way
They both closed and opened it in 5.5e.IME, bonus actions are used far more by non-casters (or half-casters) than full casters. For non- or half-casters, the bonus action becomes "here's a thing you can do in addition to just attacking", which gives them some interesting (ideally) options. Casters usually get to have enough fun with just the one spell, and have rules specifically making it hard to use bonus actions (IIRC, 5.0 allows a bonus action leveled spell and a regular action cantrip but not vice versa; and I think they simplified it in 5.1 so you can just cast one leveled spell per round no matter what, thereby also closing the Action Surge loophole).
They closed it because you explicitly can't take the Magic action via Action Surge (so that's done regardless), and they made it so you can only cast one spell that consumes a spell slot per round. You can, however, cast any number of spells if they don't consume a spell slot, e.g. from a magic item, feat, or class feature, so long as they don't consume spell slots. The Stars Druid, for example, can cast guiding bolt a number of times per day without expending spell slots.