after a quick skip tru Civ V modpacks (its been a long time) i must partially agree with
@Deliverator, overall Civ IV inspired more and diverse TCs. but i disagree with the statement of Civ V having no true (or "massive") TCs, but overhauls. Faerun is a true TC, as those r too:
https://forums.civfanatics.com/threads/blizzard-allstars.522685/
https://forums.civfanatics.com/threads/allstars-of-the-three-kingdoms.529239/
https://forums.civfanatics.com/threads/heroes-of-might-and-wisdom.509427/
so whats a true (and GREAT) Total Conversion? as explicitly stated in the intro of my OP the difference between an overhaul (enhancement etc) and a TC is not simply a diversification (or application like Sucritacts Events and Decisions) of original gameplay, but context. all quoted mods, esp. the fabulous Faerun attach a new layer to gameplay (i.e. heroes, magic, remember Master of Magic?) and alter the original context of the test of time into not just simply an alternative narrative frame, but a "different" game. FFHII may be 4X, but it is no longer a mere Civ IV mod, it rlly is worthy to be ackknowledged as a Stand-Alone, as the original Day Z mod no longer was Arma (and finally got AAA gold).
but
@Deliverator is right in his conclusion of a decline of TCs since Civ V, Civ VI not simply continuing, but finalizing this downtrend (as there r no TCs).
so the initial question still stands indicated, but unresolved - y this decline?
some hints do allow to at least speculate about a modding/TC fatigue, due to "complicated" mod support by FXS, but my personal conclusion is the overall progression fatigue in the brands development. Civ VI has been overall received positively, but many (professional) authors warned to expect the gameplay range of a fully DLC`d Civ V.
which isnt true at all, cuz Civs/leaders r more diverse, specified, as is Diplo, City management etc. - and all those changes within the minimalism redux priority, that made Civ V (in comparision to the amount of micromanagement in Civ IV, i recall a FFHII Calabim game where late-game turns averaged 30 mins) outstandingly great (sometimes less is more!). in my opinion Civ VI is the best, most iterated, most diverse Civ in history.
this fatigue now applies to an overall shift in the whole (gaming, aka THE) industry, where the influx of the next gen of gamers (and devs btw) altered the overall values to adapt to the sociocultural changes. the next gens preference r determined by mobiles (id like to reference them as the Smart Generation), Candy Crush is King.
mobile games r to be played on the go and everywhere, the average gaming time splits over the day. Pokemon Go is a premium example for a mobile game, as it blends in geotracking, focussing on the mobile aspect a true rolemodel for future mobiles.
also Battle Royal, originating of the DayZ (and deeper DM) idea, Plunkbat, Fortnite etc. as the former hype LoL - all those r quick and highly accessible games (for a short attention span btw, which is a reaction to the global informational overkill in post 2k pop-culture), as Civ (and esp. modding) require some "cognitive" (aka learning) investment.
as WoWs talent and class-balance redesign was a brillant adaption of the changes in userbase, which ensured its survival (while hardcore IPs like Wildstar failed).
also the predecessor generation (of gamers) has not simply matured (fun has no age?) as the average gamer was already 31 in 2k (and male white btw, but the industry or at least their PR press worked rolemodely hard to diversify), but shifted in consume preferences. Which also indicates a fatigue.
as the decline in TCs and overall modding indicates a loss of interest, Civ grew from a premium IP into a niche.
the brand has to do more than just Civ, Civ VII has to progress far beyond the original formulae or it may be the last release (rethorically stretched).
/edit for better understanding of the very stretched "potential last release" claim and overall reference to consume fatigue:
i played em all, from Ascendendy, MOO, Lords of the Realm to up-to-date title like Stellaris, Endless Legends and Space 1-2 etc (as RPGs, DivinityOS 2 and Pathfinder Kingmaker being noteworthy CRPGs evolution peaks), but most i wouldnt play again. i may take a retro trip to Civ3 or Drakensang, but i mastered all of them, like WoW. Game Over isnt even sad, cuz its been a good time and iam looking forward to fresh input on those, for the next iteration.
but it has to be fresh, something new. (i wont elaborate on the specifics now)
thats what i like on (Great) TCs, they r like the next step in evolution, a new, and often better version.