Apparently there are still people using it. The amount is very tiny, but still big enough to warrant it's existence.
I do believe, there genuinely are people whose machines (not necessarily related only to hardware's age) have issues with NXT, but according to certain discussion i stumbled on a while ago suggests, that majority if not even all bots rely on the java client.
Supporting NXT isn't like supporting OSRS. We are able to support OSRS and RS3 because both are Java-based. NXT would require an entirely separate infrastructure - not just the client, but the scripts, and the SDN.
I can pm the link to that discussion, if you want. It's pretty enlightening stuff, and further makes one want that Java client is retired asap.
edit
Got to ask, is Jagex aware of this? /u/JagexPoerkie