First off, modding is not "a declining thread". This is simple corporate wordy crap to fool consumers into thinking they didn't deserve any better. Modding gives you free content, usually top-notch (sometimes even better than from the original developers), from the community.
Example that quickly come in mind:
1) Arma franchise by Bohemia Interactive. Look at the recent Arma 3. Improved technology, mission editor functionality, scripting library, tools, etc. That gets curious people attention and let them unleash creativity (be it modeling, scripting, coding, design, etc). But A3 is still an alpha? That's not a factor to judge the game. Community is heavily involved in many activities that are helpful in the future. ... Not to mention Arma 2: Combined Operations mod choice: ACE2 (for extreme realism), Unsung Vietnam, WW2 Invasion 1944, Cold War Rearmed 2 (a true remake of Operation Flashpoint (2001)) and many others (huge 10x10 km maps like Isla Duala, Panthera, Celle 2 etc). It's obvious such an assortment will increase a game's lifespan at least tenfold.
2) Skyrim. Although the game was buggy at the release, community made it far better and beautiful than the devs themselves. Enhanced UI, character creations, new maps, etc. This community is ready for Bethesda's new RPG anytime - be it Fallout 4, new TES.
3) The Witcher 2 ... many other games I won't bother mention.
Modding forms loyal communities that will likely buy the next installment of any series when no drastic changes were made to core gameplay (I mean those that are perceived as negative) and the idea of a game. Pay attention - nowadays most of the series are slowly losing their identity and consumers. But there's still hope - independent developers can make a difference given their proficient understanding of game development and business culture (set up -> evolve further).
Today many corporations think community involvement isn't a big deal in terms of content. Companies love to absolute and sell the words as facts. That's stupid. They should stop being pretentious and stupid.
Looking at Battlefield 3 and how its community has downgraded within time and new releases, I invite you to explore the uncharted lands of reasoning. Why publishers are afraid to support modding?
Money money money?
Lost ability to cut content and sell it as "brand-new" DLCs?
Afraid of community to create better mods?
... other? Tell me.
Also, look at how EA are denying the existence of Battlefield 2 when presenting BF4 "new feature" Commander mode. That's some disrespectful shit and greedy culture. No wonder EA declining rapidly.
P.S.: I understand the different purpose of games. Some are story-driven. This topic is mostly dedicated to games that are combining gameplay and story or are purely gameplay / MP oriented.
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Disclaimer: This thread is obviously referring to PC gamers since console games doesn't allow or allows minimum modding capability.