Sounds to me more like he was a low grade developer with a good idea, who wrote spaghetti code that worked, didn't document it, didn't write unit tests, and didn't take steps to make his code easy to understand or extend for others. Many of these types, especially those involved early in a company, also subconsiously take steps to make themselves irreplaceable including sabotauging any attempt to get a proper team involved in the areas they coded.
It's possible to separate the idea of a game designer and a coder.
If the game was better under his direction, then he was a good game designer.
If the feature was too spaghetti-coded to be fixed, he may have been a poor coder.
But I believe a big part of the reason GvG wasn't maintained is because of challenges porting it to mobile specifically - which might be architecture related rather than code-maintenance related. And that since it was determined it wasn't going to mobile, it therefore wasn't worth much development time at all - and we got GBG instead designed with mobile in mind.
I do think some of the downfall of the game is just a fact of live service games in general - that they have to continue to evolve or they'll lose interest of the playerbase at large, and that what some people loved about them will naturally fall victim to that evolution.
It's convenient in FoE's case to say that everything good about the game was that original developer's vision, and everything wrong about it is the "new dev team" (which may in fact be multiple evolutions of what the dev team looks like). But in truth we have no idea what the game would look like with the original designer still alive.