You don't need to be a game developer; all you need is common sense. If it were as easy to make a fantastic looking 3d open world game, then it wouldn't require AAA budget and indie devs wouldn't be making isometric games.
Open world games are required to render large draw distances of interactive play space. This alone is more demanding than what's going on the screen in an isometric game. There is so much more to be rendered at any given time in an open world game, because the amount of detail is incomparable and the game must look good from every angle and distance. You can produce a great looking isometric game with limited resources and people. It's not that difficult: good art direction and style would make it look good. You don't need fancy animations or render huge 4k textures of every object in the game. In an isometric game, no one will be able to see objects up close and personal, so neither the animations nor the objects need to be perfect.
You can't say the same about open world games with realistic graphics.
I'm not saying a developer who made great looking isometric games can't make a great looking 3d open world game; they are just not comparable at all.
