Startup companies do not pay any taxes anyway. Because like you said, it takes some time to establish a profitable business. So startups could be given a reasonable period of tax exemption. That would actually encourage efficient business startups.
They would just game the ‘reasonable time’ taken to get profitable. And what bureaucrat is competent to tell you what is a reasonable time?
Do it like we do it now, with tax-loss carry forwards.
Taxing the top line is a terrible idea. Think of grocery stores with a large gross but only making a few cents on the dollar, which is a typical margin for them. They’d be paying a higher effective rate on profits than, say, General Dynamics.