Well, that may play a role, but I don't really think so...
As well made mods are immensely increasing the 'replay value' of an technically 'obsolete' game - without incurring additional profits for the distributors - it's only rational
company policy to 'encourage' gamers to shift to more 'modern' games which are - unsurprisingly - more specifically geared towards lucrative DLC distribution - Shogun 2: Total War will be a prime example, just think of all those possible, nicely portioned add-ons (Imjin war etc. ...) - DLC, which already makes up a significant share of console games-based profits.
The more the technical capacities of modders grow and the more sophisticated modding communities are releasing high-quality mods [eg. EBII, ...), the more major game developers have to think of them as economically dangerous competitors - after all, the buyer incentive 'developer-made = better quality' is rapidly losing it's edge as more and more average players are introduced to mods which are quite objectively 'better' and more enjoyable as the unmodified game. If the developers are intent to receive credits for and garner profits from their own creations rather than for distributing a game engine as 'mod basis', they'll have to 'curb' the moddability of their games...either, rather inadvertently, by sheer technical complexity and negligence (ETW) or intentionally...

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