That said, if I told people on this site that I was offering a PaaS with only 1 nine of reliability, I'd be laughed out of the park. And yet that's exactly what I'd be saying if I were to advocate for ignoring <IE10.

Sure, wasn't making any comment as to whether it's okay to ignore 10%, just that 30% <IE10 is probably an overstatement.

As for whether it's okay to ignore... I guess it really depends. I actually ended up writing a mobile web-app with Cordova, so obviously my target market had no old IE in there. It might also be true that people on old IE are people on old hardware, the type like my dad who never does any ecommerce at all, and who may not be very lucrative visitors anyway.

Anyway I wonder if someone has some actual data on this, I can make up some assumptions but I really don't have a clue. I do often read that China and Russia are big IE markets (lots of windows XP and lots of websites that still use activex stuff). It'd be interesting to see which markets are high on <IE10 and what the average value (e.g. ad or ecommerce) per user from different browsers is.

Just checked on my own website, target audience are mostly 30-40 year old moms in OECD countries. <5% IE, and virtually all IE10/11. Just 5% IE9 and barely anything below. (mostly West-European audience.)

I hardly think you're comparing anything meaningful. The hypothetical PaaS has a certain level of reliability regardless of the client your user choses to use to connect but your user's client doesn't support the server's feature set.

Anyways, I have no respect for anyone using <=IE9. I currently have to support IE8 (and 9 but that's not a big deal) because we deal with Australia and we deal with banks. At this point it's ludicrous that they haven't caught up. IE8 doesn't have the ActiveX lockin that IE6 had and current IE is immeasurably better in all conceivable ways from what's come before.