"Profitable side-project" might be an unstable equilibrium. If you're doing something without scale, it will die when you lose interest. If you're doing something with scale, perhaps it should grow into a bootstrapped startup.

inquicker.com started as a hobby / learning opportunity (2005) and grew into a side project with about $20k/yr in recurring revenue from corporate customers (2008).

Eventually, it turned into a full-time job (early 2009) and I found a co-founder (late 2009). We hired our first four employees in 2010. In 2011 we signed our 100th customer and hit $1m in recurring revenue. In 2013 we hit $5m in recurring revenue.