I don’t have evidence for this but I suspect that most users (paid or otherwise) don’t tend to use memrise for very long and therefore never really encounter this problem. To even realize that this limit exists you have to use Memrise for about a year.
Given that they seem to want to kill the user-submitted course I don’t have a lot of hope for this to change. For the “official” courses which only have a few hundred words per lesson the 180day limit is probably fine, you can probably stop studying the course after a year or so. It’s for the huge user-submitted courses with thousands of words that it’s a real usability issue in my experience.
I really want to like Memrise, I’m a pro user and I even recommended it to a few friends but it’s crazy the amount of very basic stuff that’s broken. The loading times are ridiculously long for me (maybe because I have ~12 active decks with about 15k words overall?), the UI is ultra clunky and seems to worsen with every redesign (I have half a dozen custom u-block rules to remove all the useless stuff that sometimes manages to hide controls) and this arbitrary 180day limit means that the very core of the application, spaced repetition, is strangely gimped without justification.
But apparently the priority was to redesign the whole website with this ugly yellow theme. That sure improved my learning experience!