That's not going to help.

Every time someone adds a domain to Tumblr, they'd have to re-do the certbot challenges for all 50 MILLION domains.

Plus, all 50 million domains are listed IN the certificate. It'd be megabytes worth of additional data for every visitor to Tumblr.

*.tumblr.com makes a lot more sense.

I agree with the case for using a wildcard, but just to play devil's advocate - why can't they take the CloudFlare approach?

Generate a cert for 100 of your client's domains, use that cert across those domains. Cut your 50m domains down to 500,000 certificates. Serving the right certificate for the right domain is a simple enough task.

As new tumblr domains are registered, generate more certs in batches of 100 domains.

I doubt anyone would ever seriously suggest putting millions of SANs on a single certificate, but 100 isn't too farfetched.

This seems to be how wordpress.com does it.

They appear to have a wildcard for the *.wordpress.com subdomains, though.