Feel free to also search on Google for the MSCI World Return Triangle
But you have 2 mistakes in your line of thought.
1.) The return triangle shows you the pre-tax return. To seriously compare with the loan, you would have to use the after-tax return.
2.) The return triangle shows the return for a lump sum investment. But you save money monthly. Accordingly, for each individual savings rate, you would have to decide whether to take the risk or not, because the period until the final maturity gets shorter and shorter.
From my own experience, I can tell you that this is quite simple at the beginning, when the amount in the ETF is relatively low. But when the portfolio value rises and the ETF reaches highs like just now, at least I often consider whether it might not be more sensible to shift to partial prepayment.
At €10,000 and a 50% drop... well, that's just one month's income lost.
At €50,000, a 50% drop really hurts.
My main loan is much more expensive, as I took it out 8 years ago. With almost 3% interest, which I would have to achieve as after-tax return, in my opinion the risk with an ETF is too high. Partial prepayment is done there at most.
With the KFW loan at 1.4%, I had the money invested in ETFs up to now. However, the amounts in the portfolio then became too large for me, so last year and this year I also partially prepaid a large portion there. (At that time, special repayment or full redemption was still allowed with the KFW loan)
That doesn’t mean I wouldn’t do it that way again, my head says yes. On the other hand, the fear of a crash increases with growing sums.
But you don’t have to look at it only in black or white. A mixed form – saving in ETFs and making special prepayments during market highs – is great.