> Major problem with it going offline, hence this new tool, which may or may not go offline at some point again.
I'm not sure I understand this sentiment. It's a tool that provides a capability that wasn't there before, so event if it goes away later, you still get the utility it provided while it worked. Since the cost is just providing and email address, that seems a worthwhile trade to me. If it was a trade between this and some other implementation that worked, and it's possible closure at a later date would force you to set up with another service afterwards and you wouldn't if you chose the correct service in the first place, then that's a different story.
> I am sure you could build an extension that polls the HN API every couple of seconds or so to check for replies and have it show a popup when you get a notification as well as send an email.
Well, I'm not sure it would be nice to HN to query them an extra time every few seconds X the number or people that might install the extension. If it's a query every 10 seconds, and 1000 people run it, you've just increased their load by 100 queries a second.