"Silicon Valley" doesn't get to make the decision unless they are willing to send some of those hundreds of billions to TSMC up front. (TSMC isn't going to want future promises of business either since those are worth very little.)
if what Elon recently said is true (if - but he might not be... inaccurate... on this particular thing) they already have and bought the forward production capacity of those new fabs and it still isn't enough.
And if the Big Tech companies think it is so important to get all those compute and/or memory chips sooner and in larger supply, it should be no problem at all for those Big Tech companies to pay for the costs and then have priority access to all (or their portion of) the output for the future years.
OTOH, if they are insisting on not investing their funds or stock, and it is simply pressure on TSMC to take on the risk, TSMC should be very wary of taking on risk for those players (unless TSMC sees another advantage of producing into a likely glut or supply canyon shortly after the new fabs come online).
