Would your answer add anything to the answer they provided? If so, then writing an answer is not only appropriate, but appreciable.
There was never a limit to the number of answers, and there shouldn't be. If you're concerned about the activity, commenting on issues (if any) is enough proof for activity. If there aren't any issues, land in an upvote, as a signal that the answer and/or the question is a great one. However, if you want to add an answer, you need to have something to say. You can start by:
Given that a "healthy" beta has 2.5 answers per question, I wonder if it's a good idea to postpone answering some of our own questions even if we know the answers.
Note that the 2.5 answer ratio in Area51 isn't really reliable. (Don't hang me! SE staff say it too, I'll reach it there in a moment) Here's why:
StackExchange Q & As now cover a vast sea of topics that at times hardly relate to each other; from Chemistry to RPG, from Tex to life hacks.
In a science SE community, there's usually one definitive answer to each question. Other answer are often added because the main answer missed something, not that they're bringing a brand new perspective. As a result, both Chemistry and Biology SEs have 1.5 answers per question, while any other stats are excellent.
In a language-oriented SE community, even the type of questions are a major role in determining how many answers a Q gets. A word-request is usually accompanied by more than one answer, while a simple canonical answer will do for a grammar not-so-broad question. Overall, this results in fairly lesser answers chipping in. ELL's stats say there are only 1.8 ApQ, while this isn't indicating its failure to the smallest extent.
On the other hand, a programming Q & A (or generally any topic that is practical enough for there to be different approaches to the solution) does and should get more than one answer. As so, PCG gets almost 10 answers per Q. (Some of that could be because they get lesser questions, but you get what I'm saying)
So, as you'll get some idea looking at this, the stat isn't really a "checklist" to achieve. It's simply a crude estimation of how successful a proposal has been. So, I would rather having "high quality repository of knowledge" over a site that gets answers.
Conclusion: Live your life and contribute to your site. Don't worry about the Area51 ghosts. Um, sorry, I meant stats. Think about what a future visitor will like to get: Something complete, without noise. If the answer is complete, better leave it as is. If not, then comment/add your own answer.