There is a lot of Americans who travel to Canada by air, so I would not call them a small subset. At the very least, the US government could allow the passport card to be used to return to the US by air, but for some reason they are not willing to do that either. IMO, if they did, that would prompt both Canada and Mexico to start accepting the cards at their airports POE. These two counties would cover a large portion of travelling US public. Then, theoretically both Ireland and the UK could accept passport cards too because they have their own immigration systems. Yes, I do understand that travelling to the Schengen Area would require a good old passport book, but the Schengen Area alone should not be the reason why travelling Americans would have to carry passport book for any and all international air travel.

You seem to squarely concentrate on the EU, but my comment regarding the air travel using passport card was more pointed to individual countries that have their own immigration databases such as Canada, Mexico, UK, etc. If the Schengen countries are willing to use such an outdated 'technology' as passport stumps to protect their borders and enforce their immigration laws, then more power to them.

I doubt that would ever happen in our lifetime as it would require every country in the world to abolish paper visas/stamps and adopt a 100% electronic visa and entry/exit system. The only country I'm aware of that's done both is Australia.

Lots of Americans travel to Canada by air, but I suspect that a significant portion of those Americans also travel to other countries by air and would still have passport books. I agree, though, that it serves no purpose for the US government not to accept its own passport cards for air travel from CA/MX, especially since both of those countries allowed Americans to fly with ID and proof of citizenship until Uncle Sam changed his own rules.

As for UK/IE, although I agree that they could theoretically accept passport cards, I don't see the US reciprocally accepting Irish passport cards, and the UK doesn't have one, so it seems unlikely to be something that any of the governments involved would expend much effort on.

I concentrate on the EU for two reasons:
First, it's the only large bloc that allows passport-free travel. While there are other countries with reciprocal agreements (UK/IE CTA, US/CA and US/MX land borders), anyone who travels beyond those limited areas would need a passport book anyway, whereas a Bulgarian can get all the way to Finland, Ireland, Portugal, etc. without a passport.
Second--and IMO more importantly--in most of the EU member states, it's illegal to walk out your front door without your ID. Every adult already has a national ID, so using it as a travel document relieves the traveller of the entire passport application process. The only thing Americans save by getting a passport card instead of the passport book is $8 per year and a little bit of bag/pocket space.

It'd be interesting to know how many Irish citizens have passport cards, since I suspect that there are lots of Irish citizens who travel outside the CTA but don't leave the EU/EEA.

The thing with the Irish passport card is that you can only get it if you already have a passport book, and if your passport book has 6 months validity left when you apply, so will the card.

And apparently you can't just reuse the book's photo for the card unless you are applying for both at the same time.

If I were Irish I wouldn't bother as I don't find it particularly onerous to carry a passport book around.

Australia still has the capability to stamp - and does so if Smartgate fails to recognise you.

However Australia and New Zealand are working on the concept of "Cloud Passports" - fully digital passports which will initially allow NZ/Australian nationals to travel between the two countries using no physical passport at all, but rather a "virtual" passport stored on government data centres.

I have an Australian stamp in my passport. Granted, I had to ask for it, and it's more of a souvenir, but even Australia can fall back to stamps if needed.

Outdated or not, expect increased costs for US passports and passport cards effective April 2nd for those required to use DS-11 to get a US passport. Higher price but no better outcome at least for me.