Over the long term index have historical performed well especially if your drip feeding in.
Remember the ftse100 index is roughly were it was at the height of the dot com bubble in 1999. So nil capital gains for a lump sum invested then.
Dividends are the key especially when compounding would have roughly doubled your return in that time but it is over 2 decades.
And also dividends are not consistent they often get slashed/halted during down turn or recessions, and should not be relied upon.
Consistently drip feeding in capital and sheltering from tax (via isa or pension) are key to index investing for most people.
Edit: putting into a foreign index also puts in currency risk. S&p 500 is priced in USD