I think a rational CEO/CFO looks across intrinsic/organic investments in their own company, possible mergers and acquisitions, and share buybacks and "horse races" each of those possible uses for excess cash.

Whichever combination of those items provides the best risk-adjusted return is the one (or several) that they should choose. The risk of share buybacks is quite low. You more or less know what the outcome will be, so even if the reward of a successful M&A or an organic expansion project is higher, it may still be smarter to buyback shares once you discount the former possibilities for the uncertainty. This is even more true if you believe the stock market is undervaluing your firm's shares, which of course then becomes a very common storyline when buyback programs are announced or expanded.