If we have 12 small cdns have 12 outages in a year (combined, 1/year each), each time bringing down 10,000 websites, is that better than 1 large cdn having one outage during the year bringing down 120,000 websites?
If I'm the website owner I think I prefer the latter, my customers blame the cdn instead of me. If I'm the cdn owner I definitely prefer the latter, more customers to amortize my costs over.
Obviously that isn't an honest comparison. If you're asking whether or not 10 small CDN providers could provide a more robust, higher quality service with more uptime than one large CDN provider, then I think the answer is probably "yes."
There's kind of a race-to-the-bottom[0] wrt. decentralisation.
It'd be better for the internet as a whole if we don't always pick the most popular (so when your email's CDN goes down you can still communicate on chat, when CNN goes down you can still read BBC). But as an individual I have strong incentives to pick the one everyone else picks, because that's presumably the most stable/documented/lowest cost due to volume.