It's possible, but "solo" is a lie told to keep you motivated.

Shipped a B2B vertical platform in 12 months alone (paying customers now). What actually made it possible was: three prior jobs where I learned the boring parts (auth, payments, ops, CI), domain expertise I happened to have from family (HVAC engineering, which is what the product vertical turned out to be), plus forums and OSS libraries that are effectively hundreds of past engineer-years I'm standing on.

So "alone" is really "me full-time + 20 years of accumulated industry + 10 years of accumulated open source". The actual solo part is attention allocation across features vs. bugs vs. support vs. sales. That part, yes, is brutal.

What's actually impossible for one person: doing sales at the same time as shipping. You alternate. When I'm coding I'm leaving money on the table. When I'm selling my codebase accrues bugs. That's the real solo trap.