No. You're forgetting the human element in this.You seem to be implying that this system with weakened security would be installed on every iPhone (or hard drive or file system) on the planet. That is NOT the case, but if it was I would agree with you wholeheartedly.
You may understand the tech, but you do not understand the law. The government is asking for a special OS to be developed and installed on specific phones, each of which have gone through the regular 4th amendment legal hurdles and protections of obtaining a warrant for inspection by a neutral judge. And that special OS would be maintained as proprietary by Apple, not the government. Nobody is asking Apple to weaken security on ALL iPhones. Apple can and should continue to develop strengthened encryption algorithms and protocols, but they also have to be able to comply with legal search warrants in order to assist the government in solving crimes. Developing a specialized OS that Apple maintains under their control and use only when the government has obtained a legal search warrant signed by a judge seems like a reasonable compromise.
If a specialized iOS is made, even for one device, it has the chance of being stolen, sold by employee, whatever, from Apple.
Just because Apple has control of this "special" iOS, doesn't mean it won't ever leak through theft of disgruntled employee sale. it now exists and it has a chance of going into the wild, and that is what Apple and many tech companies want to avoid.
If I may: Nobel regrets inventing dynamite. But once it got out, it killed many. Point: once something bad is made, there's no going back.