Maybe, but that’s not the issue. Many people have found that content blockers are their value-effort sweet spot.
Fallback text might actually be a more appropriate approach since it doesn’t require the user preference (and would be transparent in cases where the iframe fails to load for other reasons, eg if the src domain is blocked by the network.). If a good implementation can be found it might also be easier to maintain. An idea for an implementation: display the original URL in the post text, instantiate the onebox iframe detached from the DOM, replace the URL with the iframe once it’s clear the iframe is going to load.