And that's prohibitive? You could run a webserver from your house and set your router to be static. What am I missing?
The external IP needs to be static. Once you have that, you can access from anywhere on your home/office network. It's not always easy to get a static IP address even from your home. I had to "upgrade" to a business broadband package because my ISP didn't do static IP's on residential accounts. It took a month for them to come round and do it, and then they charged me £150 to have the engineers basically swap one identical router for another and change a couple of settings.
I'm very lucky that I own my own home, but in London, where I now live, there are a lack of houses. That means owning your own home is simply not affordable for most people (and for pretty much anyone calling themselves indie). Renting is the only way to go, but because there's a shortage of rental properties, so landlords have all the power. It's a pain in the ass just to persuade shitty landlords to do basic maintenance, never mind something like call up an ISP and get a static IP.
The other option is to get Amazon Ec2 server or Azure or something like that, with static IP, and just use that as a proxy. Apparently Sony have clamped down on people doing that.
Moreover, whichever way you slice it, it's a royal pain in the ass. It's a hurdle / barrier that I know for a fact has put off a number of indies I know from getting on with making PS4 games.