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.
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.