[neotokio]

Profits scaling were already discussed in a comment above, ofc, this assumes linear growth where substack is charging (as it is now) only on subscription without going in for revenue from promotional activities (internal promotion of content), advertisements (external sponsors), reader/creator data (google model) or digital/print publishing (leverage of user base substack reach out) in form of a (e)book. Yes, this can make profit multiplier really big (12M/yr subscription + 50x from else)

But what about integrity of service over time then?

All of this means losing initial flavor of 'doing-things-differently', a lot of other companies can provide boilerplate '-stack' for publishing newsletter (ie, recent yc news -https://github.com/knadh/listmonk - self hosted). So, in the end, the only edge of content hosting platform is how well it delivers it's content (UX, rec systems, internal ads), the bigger substack gets, the less of an edge for its creator base it will have - same as medium.com.

I risk a statement that substack is happening only because medium.com is ending.

Yes, as an author I would be interested in hassle free publishing, but after that, i am interested only in how service is helping me grow an audience, nothing else.