Isn't the problem with multicast is most networks filtering it out or not supporting it? Thus it only stays useful for the LAN network scope? So I cannot just multicast to 5000 random IPs on the internet.
Not supporting it because why though? IP multicast routing requires end-to-end state for each (source , group). Further, the boxes on the market for doing multicast (not a little GPCPU router) have limitations concerning how they do multicast within the box that makes scaling multicast a real pain (basically, internally, they will send forwarding engines traffic they don't need and choke up there because it runs out of internal ways to express the separate streams).
Some of this can be shortcut by tunneling techniques, but it is just masking the situation and adding another layer of network complexity (in an area where humans still try to manage/troubleshoot mostly by hand).
