A 50-year investment horizon as chosen by Cortana above is longer than most investors will live and be able to invest significant money in the stock market - let alone reap the fruits of their investments for early (if any) retirement.
To put this into perspective (and yes, I’ll just as deliberately be choosing my dates as Cortana):
Cortana could have been that random guy on your new friendly internet forum telling you just the same back then as he did today: that this - or any - time was just as good as any other to invest. Go all and do the lump-sum. Cause, you know, who can predict the future?
This is a popular MSCI World ETF’s performance, including dividends - on a CHF basis:
There you have it: That would have been your 9-10 years of enduring a loss in your investment (remember that you’d have to pay taxes on dividends). Furthermore, you could have lost more than 55% within the first two years.
Now, surely I have deliberately chosen a worst-case scenario haven’t I?
Yeah, I already admitted to that. It’s easy to find such in hindsight, isn’t it? Indeed.
I am not even saying this should be basis for your investment decisions - yet 10 years are a long time, and losing more than half of your invested capital can be nerve-wracking. Would you have had the confidence and peace of mind, to “just stay the course” through that?
In the end, I think it comes down to your personal level of risk-tolerance - and what you feel comfortable with. Even if “all-in” lump sum investing may prove to be the best choice under uncertainty most of the time, statistically, it certainly isn’t always. And there’s nothing wrong in giving up a few “on average” expected returns for less volatility and lower drawdown on your overall portfolio through cost averaging (i.e. while you might lose out a bit from investing a part of your savings in higher-return assets such as stocks “later”, you will likely not lose as much in case of a bear market).