HTTP/3 will not be adopted because it's a downgrade from HTTP/2 for CDN companies.
HTTP/2 has indeed massively benefitted CDNs by providing more effective capacity per buck, but UDP based 3.0/QUIC will singlehandedly destroy it, and bring it to below HTTP 1.1.
Not to mention Google, which uses it a lot for regular pages and for content (eg: YouTube video).
Adoption client-side also seems to be moving quickly too. I looked at the stats for 2 of my sites and HTTP/3 is now at 60-70%, with HTTP/2 having dropped to 25-30% already.
HTTP/2 has indeed massively benefitted CDNs by providing more effective capacity per buck, but UDP based 3.0/QUIC will singlehandedly destroy it, and bring it to below HTTP 1.1.