At the end of the day, if training or doing inference on the ML model is massively more costly in time or compute, you'll iterate much less with it.

I also think it's a dead end to try to have foundation models for "time series" - it's a class of data! Like when people tried to have foundation models for any general graph type.

You could make foundation models for data within that type - eg. meteorological time series, or social network graphs. But for the abstract class type it seems like a dead end.