I don't think copyright term is a good rubric/measure here. For SaaS, a company can keep the software locked up indefinitely, regardless of copyright term. Employees can be contractually obligated not to publish source code, even if the copyright has expired.

Amortizing development cost over the useful life of the software is maybe a reasonable thing to do (I don't think it is, but let's for a minute say I agree), but determining "useful life" is not simple.