What's the catch?

Comments on the assumption that you use the freed-up cash to buy T-bills: this is not a great idea, other than optionality (which isn't nothing).. You'll be paying more on the implied interest rate of the calls than you'll be making on the T-bills.

Comments on the assumption that you use all your funds for this, rather than just some: a problem if it is very expensive when it comes time to roll, which happens sometimes. You need to be prepared for the notion t hat a roll might consume quite a lot of cash, giving your thesis time to work out.

On the general long approach:

The leverage giveth, and the leverage taketh away.

During stretches that the stock does badly, your market-value portfolio balance will sink like a rock. This bothers some people.
During stretches that the stock is flat, you lose money. Slowly, but you lose.

When it comes time to roll your calls, there may not be suitable new options available to replace the ones you own. The implied interest rate might be prohibitive, or long-dated alternatives may not be available, or they might be banned. This is OK if the stock price is good at the time, but not good if the stock price is poor.

Your choice of underlying security might be poor. Sometimes the thing that looks safest can turn around and bite you. As a random example, GE was the sure thing in the late 20th century. Pennsylvania Railroad in the early 20th--there is a very famous huge trust that went bust that way.

Jim