Irrelevant given situation explained in OP. He said it's on auto-pilot, you do nothing, and earned a fix ROI.
If I could get more of a return on the money than I get from borrowing it, with zero risk, and no work, why wouldn't I do it? Given there's zero risk, no work and a fixed ROI, a bank'll hand you the money.
It's a poorly constructed discussion. Tons of things impact it.
If you have $100bn, a 5-10% annual average return on that is awesome.
If you've got $500, a 5-10% annual return on that is pitifully bad.
As you scale things up, you generally speaking reduce ROI, due to supply/demand curves and all sorts of other shit. ROI in itself is therefore a useless metric, other than when comparing two scenarios.
E.g. I anticipate ROI on running this campaign to be ~20% for my $10k, if I put it in the bank I get 5%. Therefore I invest in the campaign.
There's no lower/upper limit on ROI itself for taking a deal, it's all down to perspective and the vehicles available to you to make money given the cash you have available to invest.