> Also, their current attorney will probably charge just to send that letter.

Surely not $75,000 for a letter. A few hundred, yes.

Recouping even $5,000 (in a "f u, go away please" payment) would be a big win for a scrappy startup, no?

Their old law firm probably would prefer not to be drug around town making it known they gave very wrong, and very costly advice. They'd prefer the matter just went away. Those conditions are ripe for a quick settlement.