I'm the first to be against doing everything server-side (specially with no EU datacenter), but I also realize that if they want to do it that way (and it's our reality, like it or not), then they're doing the right thing by not implementing fully something that would stress the servers until they can be sure that those can handle it. I prefer that to the alternative of them deploying it fully and having lag or crashing the servers.

Anyway, what you're explaining is the same that Mr. Minagawa explained. In order to do that, you must think in the worst case scenario (5000 users needing that message, each and every one of them subscribed to a different event). That would require the server to process what info has to send to which user, and that's what he was explaining. Right now the server only handles ONE message for the whole 5000 players, without having to reserve processing resources to process and send any extra message. In other words, right now the server just looks how many PF groups there are there without looking anything else and sends the number to all clients, but by doing what you suggest it would have to look the number of PF groups for A content, the number for B content, etc. and then send those different messages to all clients. It would require the same resources as what we have now, but multiplied by n, where n would be the max. number of possible events (n will increase in the future when more content is introduced, and they have to account for that too, at least on medium term).

Also, running a website is not the same as running an MMO. The processing power and memory resources required for everything that is running here is way more than what a webserver has to handle. I know this first hand, as I've worked with and monitored both (and in an MMO more reliant on the client, that gives the servers less load than what they get when almost everything is done there as is the case in FFXIV).

It's not something trivial to do, and one has to be careful when implementing things such as those or you break the whole system.