I categorically disagree. The game is built to require some amount of ad-hoc/GM Fiat rulings.
More pertinent to forcecage, we have huge creatures (frost, cloud, and storm giants) who are explicitly too tall to fit in a 20x20x20 ft. cube, if you go by the listed heights on page 153 of the MM.
Playing a 13th Level Feylock who fights a LOT of Giants, this is always a factor. At the same time, the GM is lenient when it comes to things like giant snakes, creatures that curl up when they're ready to fight. In game terms, it means I need to ready an action to use Force Cage which is usually the case since high CR monsters tend to be big.
Back to the OP, the spells listed are not as bad as a Hold Person, Banishment, or Hold Monster with a DC19+. When my Warlock was 9th level I held Elf 4 Gladiators for nearly an entire combat. Our Druid dropped a Flaming Sphere on top of them and they just burned, auto-failing their Dex saves as we fought off the Devils. Did the Devils try to break my Concentration? Yes, they tried in vain.
The thing about those spells are they can target a creature's worst save, which could be a penalty meaning they'll never save except with pure luck. I recently Banished two Stone Giants and it cut our combat in half. Then I released them when we were hidden and ready. That's actually better than Force Cage since the Stone Giants were more than 20ft apart. Did the Stone Giants get a save? Sure, but they needed a natural 20.
Even low level spells like Bane will seem broken when they target the right creatures. Casting Bane on a large group of monsters like Trolls shuts them down because it makes their saves worse (incoming fireball) and their attacks are worse (PCs' Armor Class is effectively raised). Bestow Curse cast as a 5th Level spell has no Concentration and lasts for 8 hours. Target a Purple Worm and it's incapacitated for 8 hours.
If you're having problems with spell casters then you need to switch up your monsters a bit. Certain monsters are better for certain groups. The key is to challenge the group, but not shut them down. If everyone is a melee fighter then don't send Manticores. At the same time if everyone is a low-level caster you don't send a Rakshasa. On the flip-side, you don't send Giants to fight an Enchanter or Beasts to fight a Druid. Creatures like Demons and Devils tend to be effective against any group since they have a wide range of defenses and attacks, not to mention variety. I guess what I'm really trying to say is...
it ain't easy being a DM.