While support staff definitely increases with users (paid or not), beyond a certain size the number of engineering staff should be largely independent of user count. As long as you are able to run 24/7 ops, manage outages and incidents, scale appropriately, etc. the only variable should be how rapidly you need to build and maintain your software. The rest of the complexity is all handled via automation.
How are these two comparable? When Apple IPO'd back in 1980 you were buying stock from Apple.
>"...but they are probably not raising money because they either don't need it or believe they can get better funding terms."
They lose hundreds of millions of dollars year over year, why would they not need it? Also why would they believe that they can get better funding now when they resorted to selling debt with unfavorable term intheir last round of financing?
One of the conditions of their recent debt financing was that they IPO within a certain time frame. If they believe that they don't need additional funding at the moment or can get better terms then they are simply fulfilling criteria #2 and #3 with their unique IPO. I imagine Spotify can get decent debt financing terms b/c they have such a steady and predictable source of revenue (though not necessarily profit).