Fair enough. My preference is to just tell them that their simulacrums still count as themselves (including for casting Wish) and therefore can't have a simulacrum of their own (because if the simulacrum creates a simulacrum the first simulacrum will disappear).
I share that preference, that at my own table it has not yet come up because my players would rather be casting
prismatic spray, teleport, Mordenkainen's magnificent mansion, or reverse gravity with their 7th level spell slots.
I still think antimagic field is too small to be effective (25' diameter). Maybe if you use a chokepoint, but against an army of wizards who can just Misty Step, its efficacy is questionable at best.
EDIT
Sorry, forgot one of my talking points. Antimagic field isn't really a great way to defend against magic. The clone mage army can just use Telekinesis to drop heavy objects on you. Since you're inside an Antimagic Field, you can't even use magic to defend against those attacks.
Rearranged your post to keep the topics straight in my reply, hope you don't mind.
The effectiveness of the
antimagic field is that most of the spells a wizard, and thus the simulacrum army of a wizard, will bring to bear will not be able to target you at the center of it. You've got a point about
telekinesis still being an option, but that brings into the discussion that the field of battle must be chosen carefully, and one could limit the danger of objects by making sure most things are outside of a particular range of weight - keep things either too heavy for telekinetic manipulation, or too light to do serious damage. Plus there is the matter of telekinetic assault potentially being very slow if objects need to be moved further than a round of manipulation will allow in order to be adequately hurtful.
I think a severe alteration of your opponent's offensive strategy like that is worth calling "effective."
Similarly, I believe that dispel magic used to have an area effect option, but in 5e it is a targeted effect only. As such, it's not much use as a trap.
The area effect version of prior editions was useful, that's for sure, but I don't feel that the nonexistence of an area version actually reduces the spell's use as a trap.
For example, we can take the poison darts trap example from the DMG and alter it to make an effective
dispel magic trap to be used against this hypothetical army.
That trap consists of a pressure plate, and when that plate is activated, 4 darts are fired at random targets within 10 feet of the pressure plate.
All we have to do is change the +8 bonus to the attack roll and 1d4 piercing damage plus a DC 15 Constitution saving throw to determine full or half damage of the 2d10 poison damage into whatever level of
dispel magic casting we wish to use and the related check (if any, since this could theoretically be a 9th level casting and not need to make a check regardless the spells active on the target).
You certainly do have countermeasures at your disposal as the DM. A mage creating a clone mage army could easily draw the attention of the gods (who aren't too keen on this mortal growing exponentially in power) or the forces of balance (I don't think this one needs explaining). Of course, if you let the BBEG get away with it but punish the players when they try to do so, your response might be less justifiable...
Of course, a good DM should always make sure not to rule in any way that causes the players to feel they've been treated unfairly, since the goal is to keep everyone coming back for more fun next session.