1.) 5e casters get much more powerful cantrips than their 3.5e/PF1e counterparts. When out of spell slots, they're no longer limited to using a sling or casting acid splash for a whopping 1d3 points of damage.
2.) 5e wizards can cast ANY spell they have prepared, whereas 3.5e/PF1e wizards had to prepare specific spells in specific slots. Don't underestimate the power of flexibility - a level 20 5e Wizard can have multiple 6th-9th level spells prepared and choose the best one for the situation.
So what when you can hardly even use level 6+ spells, this isn't 'more powerful'.
Use a sling when out of spells? In BG2 you just polymorph self instead or multi / dual class some thief or fighter levels and use better weapons.
