If you follow the script, you won't buy back at $70. You immediately buy back at the then market ~$60.
* I hold the stock @ 50
* I write a call @ 60 expiring Friday
* Price goes to 65
* Expires $5 ITM
* Call is exercised at EOD Friday, now I have no position
* I have to buy at market price on Monday (probably around $65) to remain in the position
* Alternatively, I can buy back the option on EOD Friday at a $5 loss/share, which works out to the same.
What am I missing?