There are some things to consider with this scenario that you may not be looking at.
How often do you go to the doctor?
What are your COBRA Premiums?
What are your out of pocket costs with your COBRA Plan?
You can continue to make withdraws from your HSA when you switch to Medicare, you just cannot continue to contribute to the account.
As far as ...
The idea is the same as how you treat money in a non-HSA account: "What are the pros and cons of investing money in my personal checking account?" The answer is in the long run you'll likely have much higher returns, and in the short run you have a much higher risk of losing a large chunk of your money.
Similar to how you probably wouldn't invest your ...