It's worth considering the purpose of these indices as well. E.g. the Dow Jones is a price-weighted index for 30 large US stocks, taking into account splits and dividends.

A rise of 1000 in the index means it costs $1000 more to purchase that basket of shares than it cost yesterday, after taking into account dividends, splits etc. The market calls this a benchmark for the economy, which is the point: that for decades it's been accepted as an economic barometer with sustained increases representing growth and sustained declines representing the doldrums. It may be a truism that every $1000 rise in price is less in % terms, but what does that have to do with the averaging/smoothing purpose of the index?

By common acceptance, record high indices ought to be great economic news- unless you're determined to find fault. in which case mathematical nits suddenly get called into play as if they trump the market/social purpose of an index that is itself an averaging mechanism. ;-)