Sure, extensions are great, but there are a lot of users that aren't going to go through the steps to install a browser extension to use a website. Also, an extension is good for my laptop but it doesn't help me out much on mobile. 

With this setup, you are able to have the convenience of a custodial solution, but, you aren't able to get rugged if they act dishonestly. Now you can effectively rotate your keys without comprimising your npub 

 who would be the custodians? 

 Anyone who wants to run a bunker. I would imagine this will be a premium service that can be provided virtually for free. It is very light on the server.