$1M ARR and 10k+ emails? Those are rookie numbers. There's nothing special about a rails app achieving that.
You'd be surprised what kind of far worse junk than anything we're talking about can scale the same or better and is ergonomic to another type of dev. This is all just bikeshedding.
Software complexity is bikeshedding? I disagree. I think it's one of the most important things, especially for startups.
Of course Hey is still young... But I'm pretty sure you also know that they have Basecamp with 3M+ users. I mentioned Hey because I think it's a great example of sth more than lists/forms.
I think it's even less important for startups.
At some point, the business cannot be just one app or one tech stack. More devs will come aboard that disagree with the chosen tools and for very good reasons. You must work with the devs you have and the expertise they bring. Only the most out of touch CTO would avoid sunsetting legacy apps. There's the business side concerned with functionality, and then there's the hiring side concerned with implementation details. Both are key to getting the best devs and the best results.