It’s just the reality of Extension development, no one here is going to sugar coat it and say you’ll make amazing money and can live off of Extension income.
Out of the extensions that are monetized, with appropriate EBS design (or removal of a need for EBS at all) the operational costs of the average extension is not too difficult to break even. A small percentage will turn a profit but if you factor development time into it then even that may not be the case. Much like only a very small fraction of top streamers make a significant income, only a very small fraction of top Extensions make a significant income.
That’s not to say they make no income though, and many developers create Extensions as a passion project and to enhance the viewer or broadcaster experience in some way. If you’re going in to this with the intent to make money and think people will spend vast sums of bits on your Extension I’m afraid you’ll be disappointed. It’s also why some developers support their work through other means, such as a broadcaster contracting the developer to make something specific for their channel or group of channels, as that’s may be a more reliable way to monetize extensions than to rely on viewers using bits.