I can see both sides. I was a moderator on a large board (albeit not quite so big as MN) and we had to introduce charges to cover the running costs - IIRC it was £10 a year. MN is obviously a much bigger operation with staff so yeah, I can see why the costs are higher (although they have ad income, which we didn't).
It does seem a bit steep though, considering that the users generate the content that's attracting the ads and I'd imagine that the site is getting heavier traffic what with everyone 'working at home' or being furloughed, and the frothing maelstrom that is the new Coronavirus topic.
I may well subscribe for a month or so, but I see it as more of a donation to a site that's entertained me extensively over the last few years. It might have been better to offer options like the Guardian - offer both a set donation of £5/£10/other a month on a subscription basis and one-off donations. Offering a premium service when it doesn't really seem that premium is perhaps a bit of a misstep?