What the fuck is GameFreak doing? Most studios build engines and expand on them through the years. Clear examples are Valve games using the Source (TF2, HL2, Portal, etc.) and Bethesda (Skyrim, Fallout 4).
The Switch is the first time mainline Pokémon games would use a normal development architecture that isn't restricted by some handheld hardware and instead of building an engine based on it with the intention of slowly upgrading it for like a decade or so they just upscaled Sun & Moon's engine from the 3DS for SwSh. The LGPE engine was build specifically for the Switch which is (one of the reasons) why it looks superior to SwSh graphically. But instead of building on top of either SwSh or LGPE's engines they made the DP remakes from scratch since it's now being made by some literally who company and Legends's engine is clearly being made from scratch.
Even Breath of the Wild which took over 4 years of development used the Havok engine. At this point, if GameFreak wants to insist on the yearly releases, why don't they just swallow their ego and use a public engine THAT WORKS like Unreal or even Unity, and UNIRONICALLY hire that man?