Unpopular opinion - huge cheating bonuses are no substitute for an actual AI.
That's a pretty popular opinion, but an unfeasible one. I do wonder if the people making this complaint have played any other 4xes. I'm playing Stellaris at the moment. Even Civ VI's AI makes the Stellaris one look as dumb as a brick, and that's a mechanically simpler game. Every Civ game has used bonuses rather than differences in the AI to scale difficulty. Even the classic Master of Orion is unplayably easy because the AI has no idea how to design ships and its tech progression is appallingly slow (unmodded, that is).
The problem is not with the concept, it's with the execution. Civ VI, and to a lesser extent Civ V, is simply not capable of making effective use of its bonuses - so it gets big bonuses but the game barely becomes more difficult. The fundamental problem it comes back to is that Civ VI is simply too easy on all levels - if it is made appropriately challenging at the highest levels, why does it matter whether the game gets there through AI improvements or bonuses?
People who complain that the AI is 'cheating' sound as though they're complaining it's artificially difficult - if you find that to be the case, your issue is different from mine and I think from most people who are complaining about the AI. Our issue is entirely that the game is not difficult enough, 'cheating' or no 'cheating'.
I would like to see a competitive IronMan game mode as a cheap solution to the lack of challenge for top end players.
Do many people reload old saves if things go wrong? I essentially play almost every game as though it were Ironman - I just don't find it interesting reloading things I've seen before to make different decisions, and generally only ever do so if something goes wrong due to a bug.
-Deity difficulty
-Retiring a game counts as a loss. Restarting a game counts as a loss (no fishing for ideal starts)
-Random leader
-Standard size/standard speed (all minor settings standard)
-Map type cycles between continents/pangaea/fractal/Island plates (order of each block randomised).
Sounds essentially like my default - though I favour shuffle maps over fractal (which seem to produce pangeas more than anything else in my experience) and I always set map size to Huge in the absence of a random map size option (which I eternally hope will eventually be implemented). Does anyone have any clear idea whether difficulty scales linearly with map size, or whether standard may actually be more difficult than Huge (due perhaps to fewer AIs getting in one another's way, and because culture and religious victories are easier with fewer civs)?
Releasing DLLs prolonged civ5 life cycle and created direct competitor for civ6. If Firaxis has plans for civ7 (I am sure they have) they will not release DLLs for civ6.
Sorry, but I don't buy this at all. Civ V remains popular because it's a good game, but it hasn't substantially impacted Civ VI sales - Steam top sellers/played lists indicate that Civ VI is one of the platform's best-selling games of 2018 and has overtaken Civ V in players.
A wide variety of factors extend Civ V's appeal - Civ VI was frankly an atrocious game until the release of the last expansion, it lacks a diplomatic victory feature, its AI remains worse in the base game (so completely ignoring DLL-derived mods), and Civ V has been on sale frequently. Even though most of the current Civ V players seem to be people who have had the game for a while (its sale figures are much lower than its play figures for 2018), it still sold well enough to get a 'bronze' ranking on Steam, so it's still accruing new players.
You're drastically overestimating how many people are likely to use AI mods, I suspect - any of the above factors, and certainly all combined, are more likely to influence Civ V's popularity than the availability of the source code. And Firaxis is hardly likely to be unhappy with a situation where Civ VI is a top-seller and they're also still making money from new sales of Civ V.