There is no clear cut destinction, on an iPhone. It's not about dilligence, but practicality - there is no other way to wipe an iPhone but "completely" or "not at all".
Firstly, this is a complete straw man: protecting company secrets from the company mail server does not in any way require deletion of personal data on a personal phone.
The phone does not make that distinction. It is not possible to remotely wipe half an iPhone.
Basically, your entire counter-argument is a non-argument. Nothing you have said in any way justifies a company having the power to interfere with your personal data
There is no distinction on an iPhone. It does not make a distinction. Personal data is not kept separately. Company data is not kept separately. There are no divisors. There is no restriction. No sandboxing, no virtual machines. The distinction you keep making between "personal data" and "company data" is in your mind not in the iPhone data store. The iPhone data store is binary data stored in a nonsentient device which has no person itself and no understanding of personhood. There is no distinction between personal and company data in iOS. Email on an iPhone is just another app which does not distinguish personal from company messages. Your personal inbox and your exchange inbox can be merged into one unified inbox on an iPhone because an iPhone does not make a distinction between personal and work data. You can have a personal Exchange email account.