And yet they haven’t updated their pro line since 2012. And the quality of OSX is slipping badly. If they are really bereft of ideas, spending a few billion on fixing bugs (instead of the animated poop icon) might keep them occupied until inspiration strikes

But these are subjective measures (to which I happen to agree with you). It's not as if they aren't investing in their products.

Buybacks are often a signal that a company has run out of ideas for what to do with with their money, other than what they're already doing. Especially for a tech company, that's a very bad sign. For instance you refer to Apple as a mature company yet the interesting thing is that today they are basically 'the iPhone company' - making the wide majority of their profits from a product type that didn't exist 11 years ago. And this is a typical pattern with tech and electronics. Times and trends change very quickly and the winner tends to be the company that's on top of the change. If Apple doesn't manage to move beyond the iPhone they will stagnate (which is arguably already happening) and inevitably decline.

Buffett/Berkshire is literally the only example I can think of where they only buyback shares when prices drop below a certain threshold. Most companies do it regardless of whether it is creating or destroying value, based only on whether they have cash authorized to spend.

If that is a shareholder's opinion, they should sell the stock.

Buybacks tend to happen at the worst times (boom years) and cut out in recessions which is counter to what you'd want as an investor. Though if you think of it more as an alternative to special dividends it makes more sense.

It doesn't make sense for shareholders to own stock that they think will go down. If they really think that, they should sell. So shareholders should think the stock is either fairly valued or undervalued, almost by definition.

Any shareholders who don't sell during a buyback are compensated by owning a larger share of the company at what should be considered a fair or generous price from their point of view.

The place where this breaks down is when you believe a company would have a higher intrinsic value, provided that it has sufficient financing. But this isn't based on the stock price; it's based on your theory of how much money you think the company will need.

Also, from a non-shareholder's point of view (say, bondholders), a buyback means the company's ownership changed, it has less cash, and they didn't get anything for it.

Do you have evidence of this? Shareholders vote on what to do with the corporation's money. I would expect the vast majority of those decisions to be rational.

Also, as a foreigner, I have tax withheld by the IRS on US sourced income, but I do not on capital gains (except through my retirement account due to tax treaties).