By all means - let him draw what he likes. Along with "normal" art, not instead of it. But stopping making all normal art and drawing weird alien shit looks like shooting yourself in leg and that's sad because he has one of the best art styles on the market.
		
		
	 
Many artists out there changes their directions and references as a significant period of time passes. This is inevitable. TMN had drawn non-OCs for 7 years, which is quite long.
After nearly a decade, it's understandable for someone to no longer be interested in drawing non-OCs anymore. And even if you can somehow force him to draw these again, he would eventually suffer from burn-out and boredom because he has no freedom to do what he actually wants (draw OCs with world-building). In this case, he would likely announce a hiatus, depression, sadness, low satisfaction, etc., meaning that he won't be able to make contents for other potential fans AND make money for himself. So it's a loss for both sides.
By following his current actual passion (making OCs), he can keep making money (albeit less than before), while potential fans still get contents. This still gives more benefits than forcing himself to stick to non-OCs, which he now has ZERO interest in. TMN is past that stage. He likely won't come back to it.
A couple more examples. Dandonfuga, another prolific NSFW artist, had drawn for at least 6 years until 2022. Around 2023, she announced that she would reduce content while keeping same price, so subscribers are at a loss. Why would she do that? Burn-out, it's different from laziness. Doing the same contents with same artstyle for half a decade will eventually suck out the ambition from most people. This is inevitable. Want to avoid burn-out? She has to change her content and art direction to refresh her ambition. But people will complain so she sticks to her old path. Consequence is she takes frequent month-long breaks to avoid burnout, but fans get less content.
Meanwhile, Personalami changed artstyle after 9 years to anime style (which I don't like), and now is still making contents at a similar pace like before.
Conclusion, artists change their art directions for their own good at the cost of fan's enjoyment. An inevitable, but necessary sacrifice.