It's a scale. On the one extreme you have solo development, on the other you have the gargantuan code bases at e.g. Google, or the Linux kernel.
What you're describing is somewhere in the middle (if you imagine a logarithmic scale), it's at a point where working like a solo dev begins to break down especially over time, but not at a point where it's immediately catastrophic.
Startups sometimes work in that sort of hybrid mode where they have relatively low quality code bordering on unmaintainability, where they put off fixing its problems into the future when they've made it big.
What you're describing is somewhere in the middle (if you imagine a logarithmic scale), it's at a point where working like a solo dev begins to break down especially over time, but not at a point where it's immediately catastrophic.
Startups sometimes work in that sort of hybrid mode where they have relatively low quality code bordering on unmaintainability, where they put off fixing its problems into the future when they've made it big.