If you're steadily adding 100 customers/month you might think thats great because of the accumulating nature of subscription revenue - but actually that's a death sentence.
Your churn will grow as your customer base grows.
If you've got a 5% monthly churn rate then at 1000 customers you'll lose 50 customers/month. At 2000 customers you'll be losing 100 customers/month - and all of a sudden your 100 new customers a month will net out to zero. After that point you'll start losing customers.
From a quick look at Buffer's baremetrics board that's what happened here.
You either have to have net negative dollar churn (which is very very hard if you're selling to SMEs) or you have to have an exponential growth rate that means you can escape the churn effect and that almost always require external capital to fuel the growth.