Isn't this true of all new strains of all contagious pathogens? They are constantly undergoing change. We estimate low probability that mutated viruses or bacteria descended from strains we are familiar with will have dramatically more harmful long-term effects, but we don't actually know for certain that any given year's new strains won't have very different risk profiles until much later. We can make probabilistic models based on historical data, but they are unavoidably vulnerable to underestimating the risk of black swan events.

One could argue that certain features of covid make it riskier with regard to long-term effects, but that is not a proposition that is well developed in the public conversation, especially by proponents of the zoonosis hypothesis. The lab origin hypothesis with its accompanying assumptions of serial passage and direct gene modification would in my eyes strengthen the case that covid's long-term effects were less likely to conform to historical data on other viral infections, though interestingly the intersection of those who find the lab origin more convincing with those who have serious concerns about long-term harms is a pretty small set.