I guess that's the difference between our playstyles then: I don't see "turning gold into power" (Simulacrum, Planar Binding, Animate Dead, enchanting items) as an abuse of wizardly power (except maybe in a moral sense, depending on what you do with it). I see it as very close to the core of wizardly power: foresight, artifice, and preparation are what wizards are about. The cantrips and fireballs and other evocations are just thinks the wizard does to pass the time until he can level up and learn real magic, but if you have an evil wizard in control of a kingdom, you can absolutely expect him to have dozens or hundreds of fiends at his command in addition to his own personal spellcasting, and Planar Binding (etc.) are how he does it. I'm influenced by fiction such as Jim Butcher's Dresden Files and Fred Saberhagen's Dracula books (in particular the one with Merlin in it, Dominion).
Essentially, I see wizards as straddling the intermediate line between warfare-as-individual-personal-prowess (default D&D/Fighter paradigm, limited by your own personal HP) and technology-driven industrial warfare (limited by the raw materials you have to make bombs/tanks/bullets). Unlike technological warfare, wizards don't get to scale out by building factories to make factories to make factories (personal spellcasting ability is a bottleneck), and also unlike technology they also have some intrinsic power independent of any logistic pipeline (can throw Fireballs using only spell slots, unlike an ultratech general), but they're about halfway there.
RE: Banishment, it doesn't actually break a Planar Binding (so you could just re-summon the Goristro and it would still be bound). Also it works almost just as well on Prime Material creatures (especially wizards, who have terrible Cha saves) so I'm a bit surprised that the enemy spellcasters aren't indiscriminately Banishing PCs as well as fiends, but in any case it still takes ten Banish spells to banish 10,000 gp worth of air elementals, and the elementals (and PCs) can inflict a lot of damage while those Banish spells are being cast. Planar Binding just one elemental is kind of pointless though, like bringing along a pistol with only one bullet in it: why wouldn't you load at least six bullets?
Did I mention yet that none of my PCs are at a level where this would matter for them yet, and that the first 5E wizard in my campaign to (ab)use Planar Binding will probably be an NPC villain? Your Underdark campaign is your own, but if there are wealthy and powerful enemy spellcasters everywhere in your campaign, I personally would be less frightened of Banishment spells cast in person by NPC villains, and more concerned about the possibility that at any time someone could scry me out and then open up a Transport Via Plants in my vicinity and drop forty Air Elementals on my head with orders to kill me[1]. Sure, it costs 40,000 gold pieces to arrange the hit, but we said "wealthy and powerful" enemies, right? Banishment isn't going to help me survive that unless I manage to Banish myself. ;-)
[1] Alternatively, the enemy could travel to another plane and then Gate me into his presence where all the elementals/demons/whatnot are already waiting.
How are you summoning a CR16+ outsider and binding it? Planar Ally? I'm sure there are some caveats around that. Or are you hitting up your 17th level Wizard buddy for a free gate spell?
Contrary to disbelief I'm not nerfing my Wizard player on purpose, he himself has deemed all this rather ineffective and a waste of time and resources, quite simply due to the fact I have used EXACTLY these tactics against the party and they were less than effective.
Multiple elementals? Banished (you can banish multiples at higher levels). Big scary demon via planar ally? Banished, here's your free XP.
Yet somehow when the DM uses the same tactics as the player he is "punishing" his Wizard or "doesn't understand" the spells.
My Wizard player can simulacrum whomever he likes, but he has to go through the process in game, which unless is someone in his own party (pretty ineffective), it's a bit more complicated that working out some potential DPR on paper.