there is a time of comming and a time of going. As someone who knows how demanding that job can be (did it for 12 seasons) 4 seasons gives you a good prespective of what you are doing. Also, you were forced to go through somewhat of a rebuilding period. Our "golden" generation was running on it's last legs. you had to bridge a period hoping for better to come around in the near futur.
Despite that, your results have been nothing but spectacular! you narrowly missed the 2nd round of europe with an aging roster. you missed the KO stages in your first consolation tourney after facing 2 CT's (Wales & Dominican republic) & a loss to a team who should be at worlds (Spain). An argument can be made that you could have forseen one of those CT's, but consolation tourney is a lottery and you didn't get very fortunate in yours.
Last season, you were once again close to making the second round. This time, it was a 30 point difference which meant you weren't going. and then during this season, you seem to have made 1 mistake (against Singapore).
So if i summarize, you have kept Belgium in the european subtop or as close as it can be. You seem to have made 1 tiny mistake, which is a lot less than i did in during my time as NT coach. All in all, you can reflect on a very good tenure! You did our nation proud & you did it with a smaller talent pool to select from.
The belgian NT job isn't a very gratefull one. Not selecting a player often earns you a negative review. And even if you preform as well as can be expected, you still don't get any or hardly recognition. so don't worry about a dwindeling popularity! Add to the fact that the current format is very unmotivating imo (you should be able to see a post on the U21/NT fora of a suggestion i did to change the format) and i can understand while you would be calling it quits. i just hope you'll be able to still do some work at a U21 or a NT lvl!
Congratulations on a good run! If anything, you should be proud of your results!