This has crossed my mind a lot of times and I think the comment is right - market capitalization of index cannot increase to infinity. There is a finite number of companies that the Index is comprised of and there has got to be a theoretical upper limit to that.

Moreover, the investment psychology also comes into play here. The growth rate of index may slow down as it becomes large because investors always compare today's index with the previous values and are likely to conclude that the index is "over-valued" if it keeps rising.

I'll be curious to find evidence of deceleration as the index value rises.