"But without a "legacy IT" constraint, what's the compelling spreadsheet model for a YC company to choose a mainframe?"
The most compelling reasons for the mainframe, based on its intrinsic values, are 'RAS' (reliability, availability, serviceability). However, those virtues come with a very high price tag. So far the most part, I cannot imagine any startup business choosing one (UNLESS system downtime will kill the business).
Plus, you can squeeze out those same RAS factors from a cluster of commodity x86 boxen, at the software level, if your devs are willing to learn a little Erlang.
The most compelling reasons for the mainframe, based on its intrinsic values, are 'RAS' (reliability, availability, serviceability). However, those virtues come with a very high price tag. So far the most part, I cannot imagine any startup business choosing one (UNLESS system downtime will kill the business).