That is entirely irrelevant to what I posted, so let's back up and ignore these last few posts. That you featured the Archmage NPCs (perhaps the most hyper-specialized monster stats block of all) and not just one but two, and furthermore took the time to customize them, do not change the general picture.
It was way more than just two archmages.
My players have gotten good at working together. Additionally, we play with feats and multclasses and more magic items than seems normal for 5e because that's the style of game my players prefer. With six 13th level characters it took a heap of monsters to challenge them. On top of the archmages, there were multiple iron, stone, and flesh golems, six werewolves, several mage and lower level wizard NPCs and a bunch of non-spellcasting fighter type NPCs. Total monster XP for the area was a little less than 200,000. The "dungeon" was handled with one short rest mid-way through and the monsters used that time to adjust their defenses and tactics based on the abilities the PCs had exhibited.
"The PCs at these levels are packing some major powers and high damage so the monsters would have needed high power of their own to handle them."
You are correct - high level PCs in 5e are incredibly powerful and bring a variety of resources that makes them very hard to challenge. In comparison to everything else PCs can do by the time they have 7th level spells, forcecage is not overpowered, unless you define all 13th level PCs as overpowered.
I do agree that the bog standard monsters in the Monster Manual are weak compared to a party like mine. A baseline group of four PCs with few magic items (as seems to be expected by the rules) and no optional rules (e.g., feats) in use might find standard MM enemies to be a good fit. I don't have any experience playing at that baseline. I enjoy tinkering with monsters so players who have the MM at hand are wasting their time looking up stats during play (a common occurrence since we play online - I can't confiscate books).
Specifically discussing forcecage, it has quite a few limiting factors. Size is the first and foremost limit. Being 7th level is a big limit. Short of 20th level, nobody gets more than one 7th level slot, even at 20th level a PC is limited to four 7th level spells (using all 7th, 8th, and 9th level slots). That's a big resource cost for casting forcecage.
If you want to talk about a spell that is easy to categorized as overpowered, go for banishment. The number of big monsters that my players have killed by banishing and then preparing big reaction attacks for when the caster drops concentration is daunting. Banishment doesn't have a size limit and uses a Charisma save (almost always bad for monsters) and a 13th level wizard can cast banishment seven times a day with four of those taking out multiple monsters per casting.
