To be fair, a lot of people keep a digital sabbath or something similar every week and they are not even Jewish.
To me, there is one reason and one reason only to stay on the derech - you believe in hashem, and want to follow in Hashem's path.
Any other reason to stay on the derech is superficial - if someone is truly an atheist - however much you agree or disagree with their opinion/lifestyle - having them keep shabbos or kosher for some superficial reason is kind of silly. Does kosher truly existing in the absence of belief in Hashem? If you don't believe then kosher is just some random diet.
As for those who believe but go OTD, some of the reasons to stay might be reasons to leave - for example, some might suggest that you should stay OnTD to remain close with your family, but maybe family members are the reason that they went OTD. And I could say to them - "You still believe, maybe you don't want to go OTD, but stay OnTD in a better environment".