The oxygen regs are IMHO a fait accompli because - in this bit of Europe - you can't go very far anywhere near FL100 or higher without going IFR i.e. needing an IR.
And the interaction of viable Eurocontrol routes and cloud tops / icing conditions tends to mean that IFR flight is done at higher levels still. I personally don't even bother filing below FL140 in most cases; one can ask for a "stop climb" earlier if it looks good.
I know there are IR holders who fly IFR without oxygen but I find it hard to understand why someone would go all the way to getting an IR, getting a decent plane, and then chuck away half one's operating ceiling by not getting a portable oxygen kit for about £500. I had oxygen for my earliest VFR flights down to Spain... makes planning so much easier because one can fly above various military areas.