It's certainly not a hard goal -- I wouldn't pretend that even SaaS businesses are all that similar. (The degree of reassessment after 3 months is up to you, after all.)

It's really a way to frame the product launch by forcing you to ask hard questions about what you're doing. If you don't even see a way for the product to reach $10k/month with a short-term plan, maybe it's a side project rather than a startup? Or maybe the product niche is too vague or unprofitable, and you need to work on that first?