I highly doubt that the return in investment was seen immediately for personal computers. Do you have any evidence? Can you show me a company that adopted personal computers and immediately increased its profits? I’ll change my mind.

I know people who bought a computer and automated away a massive amount of work and thus paid it back in a single day in the 70s.

Back then companies needed a massive amount of people to sit and do all the calculations to do their accounting, but a single person using a computer could do the same work in a day. This was so easy and efficient that almost every bigger company started buying computers at the time.

You don't need to automate away accountants, you just need to automate away the many thousands of calculations needed to complete the accounting to save a massive amount of money. It wasn't hard to convince people to use a computer instead of sitting for weeks manually calculating sums on sheets.

I'm sorry but you're asking me here to dig up decades old data to justify my claim that "The spreadsheet software has an immediately identifiable ROI".

I am not going to do that. If you won't take it at my word that "computer doing a worksheet's of calculations automatically" is faster & less error-prone than "a human [with electronic calculator] doing that by hand", then that's a you problem.

An apple II cost $1300. VisiCalc cost $200. An accountant in that time would've cost ~10x that annually and would either spend quite a bit more than 10% doing the rote work, or hire dedicated people for it.