The irony is that nobody was as much a hereditary elite as the Roosevelts.
It could be that TR thought that the inheritance tax would only hit massive industrial or corporate fortunes like Rockefeller's, Harriman's, and Carnegie's and leave older elites with their smaller fortunes intact.
If Theodore and Franklin believed that their background and position in society made possible whatever they contributed to the nation it seems a bit strange that they'd want to liquidate the fortunes that made their achievements possible, but maybe they thought their children and grandchildren would easily find careers in the growing government sector or elsewhere in the economy.
In any event, none of the subsequent Roosevelts rose as high as TR or FDR. FDR's kids more or less drifted after he died.
You’re probably right in the sense they may have figured their heirs would have an easy time of it; this really was a land of opportunity a long time ago.
Some responding to my post seem to think I’m advocating for a 100% death tax; I said no such thing.