When they became effectively impossible to audit. That is, at their inception.

I've once worked with a trader whose magic sauce was encoded a spreadsheet he had been developing and carrying around for 10 years. It had tens of thousands of formulas and references cross ~50 sheets.

He suspected he had introduced an error somewhere, but gave up after a couple of months of trying to, you know, verify the whole thing.

Yeah I'd say that If you have a hundred formulas and more than 10 sheets, it is time to change to another tool. But when someone is this far, they'll have accumulated so much technical dept that they will probably have to hire a programmer because the typical SASS won't solve all of their needs.

When each user has a different copy, and nobody knows what the most up to date version is.

When you have error-prone processes that you do hundreds of times a day based on those spreadsheets, that a bigger application can automate.

When ten of those spreadsheets could really be replaced with one.

As with most things, its strengths are its weaknesses. Versatility is great, but it's also the enemy of things like standardization, data security and data accessibility. The more people are collaborating, the more those things start to be important.