AT1 FY21 - General Discussion + FA, page-919

Opportunity cost exists every time you make an investment decision, let's say you invested in company A rather than company B, your opportunity cost is the potential benefit from invest in company B. Vice versa.
Meanwhile, if you decide to hold company A, the opportunity cost would be the potential benefit from investment for company C, D, E, F, and so on.
It's always there, even if you not thinking about it.
When people say opportunity cost is huge for AT1, is because during the period since IPO, most other stocks have gone up, but this has gone down. People lost their potential benefits from investing in other stocks because of buying and holding AT1.
It's true that people invested in AT1 have lost the potential benefits from not using the same money to invest in Bitcoin for instance, but the question is, why people did not invest in Bitcoin in the first place?
It's all easy to say things from hindsight. Nobody knows what's gonna happen, it's all about decision making.