Each period of 15 years, in a completely even World XI, would see 1.23 cricketers picked, so he's perfectly within his rights to pick just the one.
Playing more Tests per year does not mean the upper echelon of cricketers are dominating cricket more than they did in previous years. That's what being a great is all about in my books - being a lot better than the average cricketer of your time. The more comparatively better you are, the greater you are and the more you deserve selection in something like this. Whether or not cricketers would perform in other eras is entirely irrelevant because that wasn't their job or what they were trying to achieve. Sydney Barnes's coach wouldn't have been saying to him "No, stop bowling like that - what you should be doing is bowling in a fashion that will be effective 100 years down the track when you're long dead, even if it means you'll be far less effective now, because that's what really matters". It'd make as much sense as telling Dale Steyn that he should be bowling side-arm lobs because they would've been more effective in the 1910's, or disqualifying him from your World XI because he wouldn't have been as good then. Their goal was be as effective as they could with what was in front of them and yes, that batting was easier in some eras and bowling was easier in others is something that should be very heavily considered, but you can't just wipe off an entire era as easier in general for everyone, because it doesn't work like that. Your value is measured by how good you were compared to everyone else in your time.