Medicare Registration at 65 - No Longer An Option

I was recently forced into retirement, at 68 years old. Until this time I had corporate-based medical insurance, so I figured there was no reason to register for Medicare until I did actually retire, which I was planning to do when I turned 70 (I loved my job). I did receive notifications suggesting I register for Medicare, but ignored them as long as I had corporate-based insurance from one of the top-3 providers. 

It turns out, that it is not really an "option" to register for Medicare as soon as you turn 65, but in reality, in the eyes of the top-3 medical insurers it is a requirement! These days they deny full coverage, and only pay "what Medicare would not have paid" even though you continue to pay the same full price for insurance from them that you did before you turned 65, in addition to what your employer is also paying on your behalf. 

This doesn't seem fair (nothing in life is fair) as they do not reduce your premiums, while they drastically cut your coverage without prior notification. Additionally, all of the Medicare-registration notices I received did not state anything about private corporate-based insurance would take this path of drastically-reduced coverage and the expectation that Medicare would be your primary coverage.

Shouldn't the insurance companies boldly state the fact that at 65 they will no longer act as your primary provider? Shouldn't the insurance companies boldly state the fact that at 65 they will continue to charge the same price for your insurance premiums even as they no longer pay as your primary provider? Shouldn't the Medicare notices boldly state the fact that at 65 it is not an option to sign up, but rather a requirement even if you have corporate-based private insurance?

What do you think?