Well, 7th level is a bit up there, but it's still lower than 8th + a dispel, or a half dozen disintegrates and fireballs it would take otherwise. And forcecage is a spell we know V has, is evocation (a free slot) and is probably the most powerful spell for that entire level. The duration, lack of saving throw, and immunity to dispel are all insanely good features. It's one of the most broken spells in the game. It costs thousands of gold to cast for a reason.

Also, it hardly takes a powergamer to make a wizard overpowering in DnD. The game isn't balanced at any level of skill. It doesn't take a power gamer to realize that Fly will be really useful for not getting your face stabbed, or that even weakling critters like summoned monsters 1 or unseen servant can trigger traps for you, or that given a relatively short amount of time compared to actual labour, Wall of Stone and Stoneshape can be used to build an entire fortress, accessible by flight (or even dimension door/teleport) only no less. The list goes on. My personal favourite being Rope Trick. That spell should be 9th level, not 2nd.

And remember, you're comparing this to an unoptimized Fighter (or worse yet, Monk or Barbarian) who's skillset includes doing things a commoner can do, and hitting stuff as long as it's AC isn't too high. You know when you're playing a video game and keep asking yourself "Why doesn't the villain just not open the door? They have no way to get in otherwise." Well that only applies to fighters. They get stopped by doors, holes in the floor, walls, monsters made entirely out of stuff you can't hurt with a metal stick, things that move faster than them and have a ranged attack, things that can fly, things that are invisible, bodies of water, lack of food, long distances, and worst and most common of all; other fighters of comparable level in slightly greater numbers. A wizard is rarely stopped by any of those things for more than a round or two, and almost never for more than an entire day to memorize -a- (not 'the', there are many options) correct spell.