> Can the public markets pony up the cash in the short timeframe?

Yes, very easily, American households alone plop a few hundred billion to over a trillion dollars into the stock market every quarter. Whether investors want to is another question. (The answer, at least to the tune of $75bn for SpaceX, seems to be yes.)

What I mean is that investors don't want to pony up the cash at current market prices. They want a discount. Then they will pony up the cash

Just like creditors demand higher rates on investment grade bonds, investors are demanding a higher risk premium if they're going to be expected to keep piling in cash to this particular sector that's diluting and raising.