I guess that's a question of what the purpose actually is. If you only take out $50k, then you're living on $50k that year and you pay the same amount of tax as if you had worked a regular job for the same amount. If you want to live a more extravagant lifestyle, you take more out and are taxed more.

This could mean that they have $1 million of unrealized gains that year. You're not really able to tax unrealized gains most of the time because they could disappear or go negative. That would go untaxed until they sell, but it would eventually be taxed.

At the same time, they're not going to be able to enjoy their wealth only taking out a small amount every year, so I'm not entirely convinced that this behavior of living frugally to pay less tax is something we need to 'prevent'.