If you buy shares from anyone, either they were comfortable with their previous asset allocation (and will therefore seek to re-invest that money in some other stock), they were selling shares to rebalance their portfolio, or they were selling shares as part of their planned selling to fund their retirement or meet the obligations of their pension payouts (in which case that money will be spent in the economy).

It's only in the middle case (which I think is the most rare of the three) where the share buyback money ends up as savings. (And even there, it's likely in government bonds, lowering the cost of government borrowing when done in aggregate.)