irredeemable bonds

Sir I know I have asked this question to use in the past also, put somehow couldn’t find that same post, to understand it deeper still.

So i had asked you about why do we have MV as denominator for irredeemable bonds, as our coupon payment x(1-T)/Fair value is our actual cost of borrowing from public rather than coupon payment x(1-T)/Market Value

And then you had suggested that it becomes the cost of borrowing for the firm(rate of coupon) if it wishes to float its new bonds, now, is that right?

So for example our 10yr bonds floated at $100 face value and we offered 5% coupon payment. If 4 yrs later they are trading at $80 then does that mean if company has to issue more bonds then it has offer 5/80=6.25% coupon on face value of $100 (for the company the net cost will be 5*70%/80=4.375% is a different story), right sir?