I agree this is a problem. I suspect something like this was the cause of a forum problem (and confusion) here:
See my later response on that thread to that comment…
This was occurring before 5.3, where the user downloaded a Workflow containing a component that utilised the Continental Nodes. From what I can work out, they had subsequently installed the missing extension but the Workflow was irreparably broken when they saved it. This then made it appear that the component itself was the cause of the error, which it wasn’t (KNIME had broken it). They had to redownload the Workflow but had this been their only copy that would potentially have been a bigger problem.
I would expect missing nodes to remain as connected “placeholders” awaiting the extension and then work once the extension was installed, but this doesn’t appear to be the case.