The people who come up with 409a prices have every incentive to make it as low as possible provided it is somewhat defensible to the IRS.
I assure you they can get more creative than saying that the last preferred price was at $X, therefore our hands are tied and the common must be close to that. They can take into consideration the preferred preferences, the current state of the business, the time since the last round, etc. For example, the 409a value can keep going down and down if the value of the business is (defensibly) going down and down, regardless of the last fundraising round.
This is a thing I'd love to see data for - the strike price discount at time of acquisition for later-stage companies. These same companies in that megaround companies probably have stock on secondary markets, which might be a good proxy for some of this.
And totally agree wrt creative arguments being viable... Just not clear what ends up happening in practice. Ex: I can imagine a split between paper unicorns vs ones w revenue backing it up being closer to market, and those later ones often switching to RSUs. So genuine curiosity here.
I assure you they can get more creative than saying that the last preferred price was at $X, therefore our hands are tied and the common must be close to that. They can take into consideration the preferred preferences, the current state of the business, the time since the last round, etc. For example, the 409a value can keep going down and down if the value of the business is (defensibly) going down and down, regardless of the last fundraising round.