+1 for this. I opened https://support.zabbix.com/browse/ZBXNEXT-3050 a few months back. I'm happy to sponsor a mirror. Offering rsync would allow everybody to also create their own private mirror.
1.) Disk space requirement
wget ‐‐execute robots=off ‐‐recursive \ ‐‐no-parent ‐‐continue ‐‐no-clobber \ -l 100 --reject-regex "=" http://repo.zabbix.com
=> 9.2GB (html files included) ~= 10GB
BTW: 10GB of AWS S3 standard storage (99.99% availability, 99.999999999% durability) = $0.3/month
Only traffic is a question.
Well, 10-20 GB of space and 10-30 Mbps of traffic I can donate as a person, not as company (on kimsufi, but this is enough). 
I believe we can have better availability if we make this the same way as httpredir.debian.org is made: round robin DNS and a swarm of backends.
Another advantage: if company uses Zabbix repo a lot, making public mirror and using redirector makes perfect sense - their machines should be pointed to their repo. If it is running, but when it isn't, packages still will be retrived from another repo.
Another advantage: better geographical coverage.
I know, we can make private mirrors, move to servers with better availability etc. but if we are changing things, why not make it really good? Especially that not available repository is PITA when things are automated.