I don't agree with the multi-tenancy. I've been part of two start ups that cracked the 3 million user mark (from staff to 3 million +). They all lived happily

I don't agree with the multi-tenancy. I've been part of two start ups that cracked the 3 million user mark (from staff to 3 million +). They all lived happily on one postgresql database. We didn't even consider breaking the DB apart until near this point. When we did we had the income to support the staff. I'd say don't build for scale. When you need it, you'll be making enough to pay for it. Even if you have to start over, you're income will pay the devs.