Yes. But if they're not paid subscribers, there's an opportunity to upsell. If people forget about reading your blog, they're unlikely to want to pay for it.
Also, if they're regular readers then they're more likely to share links elsewhere on the Internet.
I have a Substack with nearly 700 subscribers. I don't even have an option to pay for it. Nothing about the email feature increases the likelihood of Substack making money from my subscribers.
Like you, I have no plans to charge money for my blog. Substack doesn't seem to care about making money from us.
But a lot of Substack's features are targeted towards professional writers who make money from paid subscriptions, giving Substack a cut. We're just along for the ride, so we get those features too.
Also, if they're regular readers then they're more likely to share links elsewhere on the Internet.