The risk I see for Substack is that if a writer has enough ad revenue in parallel to their Substack subscriptions they are more likely to jump ship, or at least

The risk I see for Substack is that if a writer has enough ad revenue in parallel to their Substack subscriptions they are more likely to jump ship, or at least, threaten to reduce the 10% commission. I haven't used Substack's creator software so not certain if they offer more value than a payment process (or even a vertical-bank designed for creators) would. But neither of those would charge 10%.