Short rejection after two years I've submitted a short paper to a well-known cs journal. It got rejected after two years with the below one para explanation. While I admit that the submission was not strong, I feel cheated because of waiting for two years for this explanation (no full report was made).

In the paper, the author shows that NP-completeness for [...].
The proofs are very basic and standard. The results are reasonable, so it may be acceptable if this paper was written in 1990s. However, as far as I
know, such a kind of paper is obsolete nowadays. Since 2000s, among
such papers, few papers are acceptable for [...] only when there are
some extra value to be published. For example, they solve big open
question, or the NP-completeness has very important influence to the
other research. From this viewpoint, this paper is not strong enough
for the recent [...]. So my recommendation is reject.


  What shall I do? I think this situation is unacceptable, let me know if you disagree.


I've already complained of course to the editor upon receipt of the report, but I got no answer.

ps. Btw, imo the given journal still publishes NP-completeness proofs regularly, that's why I've chosen it for my result.
You seem to mix two different aspects: Your (understandable) disappointment about the response and the long time it took to get the response. 

You cannot really complain about the response itself because it generally seems to be rational and the reason for the rejection is not the long time it took. Right now you can only try to use the helpful aspects in the response to improve your manuscript.

On the other hand, you should complain about the time it took. Two years is indeed inacceptable. If you will get no reply, there is very little you can do, I'm afraid. It is probably best to look for an alternative journal.

My advice for the future: Do not submit to the journal again, but count it as an important experience: next time, you could ask in regular intervals (not too short or long, let's say every three months), and also give the process a maximum timeframe.