500k a month to support 19 million person play base doesn’t seem totally unreasonable. They’ve already made £400m+ in a month in early access in the first month - so it’s a drop on the ocean at the moment.
Costs will probably come down - at the moment they’ve been scrambling to keep up with demand which means expensive rapid deployment rather than long term server build out.
And presumably they plan to get the game out of early access so potentially get more players (although may not get many more players in this case as it’s so popular) and more importantly start rolling out DLC content to make more money.
I doubt they need to go the subscription route plus may be too late as they launched without it.