Going off the other reply, I wonder if a highly-active secondary market means that companies can raise series [A-Z]+ rounds effectively forever, where each "round" just refers to a giant purchase of shares under strict company supervision. Is this the new game for startups?

If investor pool becomes too loose the company becomes de facto public, subject to all SEC-enforced regulations.

The judgment is subjective though, so pushing the boundaries could be a calculated risk.