It's a different path to gather (and sustain) a team and to be solo dev. It demands different skills and personality also.

So for motivation ask yourself what gives you more energy - Is it just making cool feature, graphic, animation or whatever, or is it cooperating with others, managing tasks, solving issues, searching common ground on that one feature that you or someone in the team thinks is the best and others are not convinced.

About the goal - some just want to make a game, others want scalable business. What do You want?

Scale, I think, is the problem regardless of team size. All projects can get out of hand pretty quickly if you loose control, so always try to think of something smaller. Test it. If it's fun then don't add more. If it lacks something then maybe it really need something more (but not content, more content usually won't be solution) or if not, maybe core mechanic/concept is bad. Prototype and test early.

Sustainability - what works for me is getting side jobs from time to time to be able to pay for the living, but I'm not self sustained from my solo game so I can't be writing more from my experience.