The account registrations being disabled is likely intentional, so isn't something to "fix", at least, as far as Nyaa.si owners are concerned.
What should be fixed is the messaging - clearly "temporarily unavailable" in not the case* and it'd take like two minutes for someone to replace it with a clear, accurate message. I've seen plenty of people assume "temporarily unavailable" is accurate, wondering when registrations would be open again.

<rant>

Which highlights the key problem of Nyaa.si - the owners simply Don't Give A Fuck. They've got no reason to deceive, and it doesn't appear that they have some distaste towards newbies (other than not wanting to deal with them), they just simply don't care about the site they're running.
No one should be surprised if Nyaa.si just suddenly disappears one day, without warning (just like what happened to the original NyaaTorrents). You might not want to hear this from someone that's shutting down their own site, but I don't see Nyaa ever posting about their decision to shutdown before it eventually happens.

And disabled registrations doesn't "just work" - not allowing new contributors is a broken system. The effect clearly isn't immediate, it's just something that causes a slow gradual decline. In communities, people come and go, and if you prevent people coming, you've just got the natural attrition of users leaving.

People really shouldn't be content with the status quo because "it works". Not only is this an incredibly low bar to set your standards at, if this was universally accepted, there'd never be any improvement.

What's unfortunate is that it's hard to get attention to problems until they actually blow up. I can't say that Nyaa.si will ever "blow up", but in my opinion, the community's over-reliance on one site (whose owner hardly cares) is a problem that too many are apathetic towards.

I'm really hoping that nekoBT can eventually be a worthy alternative. More options is a benefit, not a drawback.

* I'm aware that the top of this page has "temporary" in the description, but it's at least qualified with "somewhat", and ultimately no-one cares about the precise message here anyway