You guys don't like bomb every series you're reading? C'mon, it's a nice gesture! I only do empty subs when I'm in a real hurry and can't read any at that time, but I try to return to it ASAP.
The best you can do is the same method people are forced to do on Youtube - every now and then, ask for people to leave a like, ask questions in the description so people will have an incentive to post, ask for them to share it around on social media.
Unfortunately, Webtoon created a culture where people just consume content and run. You can't change everyone and trying to force people to change only turns them away, so gotta try and catch the view that just forgot or are in a hurry.