I can see that not everyone likes Cloudflare, but generally a CDN that allows cheap delivery of the bandwidth heavy parts (e.g. images and scripts) makes probably more sense in reality than P2P.
While the P2P idea is very nice, most residential lines are heavily geared towards download speed. Upload speed is often only 5-10% of the overall speed. Using a P2P network outside of your Lan will probably lead to a slower experience than using a CDN that has PoPs very close to you.
Now I'm pretty sure that Cloudflare would save Wikipedia at least 95% of bandwidth with the free tiers. And I don't think that Cloudflare would complain, the gain of having them as clients would be larger than the costs for bandwidth. Similar for other providers, Wikipedia and their traffic levels would probably get very cheap offers.
But I can also understand that it would somehow question the independence of Wikipedia from corporate interests. That's why they didn't do it.
Yes, it actually is, I use it for a simple website I use. It actually decreases the load on it. But, you lack a lot of features and you can increase it. It cost like $20 or more per month up into $100 per month it depends I think. But, they probably don't want to go this route.