Eventually someone is going to figure out how much money they're making in their market. Maybe someone they bragged to at a mixer, or maybe an employee of one of their customers.
It's very hard to stay competitive when you have nobody to compete with, and expanding your product into new markets can, perversely, give you competition because you paint a target on your head (hey, we do X, and they're taking our X lunch, we should start doing Y and see how they like it).
Maybe a little bit of growth would be good for them. Me, I like about a 25-30% growth rate a year. As a lead dev I've always struggled to keep up with the changing team dynamics when it's bigger than that, and most of my peers have had similar experiences, differing only by degree.
I don't think I've ever seen someone manage more than a 2x growth rate without the wheels coming off, and yet I've seen many try 5-10x and flame out in the process.
So for them to ramp up without growing pains, they could easily add one person this year, another in about 8 months, and another 6 months after that. As the team gets bigger you can hire people more often, because you're used to it and your protocols for absorbing new people get better and better. Just remember to assign some of those people to trying to do more with what you already have, and identifying frugal expenditures that have multiplier effects (I've had to beg for $5000 in hardware and gotten another FTE instead so many times it's become a cliche). The working smarter aspect will keep your team lean and build a lot of confidence they'll need when a competitor or a fractious customer finally shows up.