Stock buybacks are the use of corporate cash to buyback shares and hence reduce share count. They may well increase the price of a given stock, but they are market capitalization neutral hence they have zero impact on stock market indices. They don't increase the size of the pie, only the size of the slices.
By no coincidence, corporate cash is at a record $1.43 trillion, so 84% of current cash is earmarked to be liquidated. And the last peak in buybacks coincided with the 2007 peak.
ZH: March 6, 2015:
Insider selling by Tech Executives highest in 8 years
At best, stock buybacks are market neutral, due to share count reduction i.e. they are an unsustainable gimmick that reduces cash available for re-investment. Stock valuations represent the present value of future earnings, so if those are falling, no amount of buyback chicanery will save the market.