If you spend vast amounts of MP on culture change it costs you.
Absolutely.
Again I have no problem with people making choices that are not historical because the whole fraking point of playing EU4 is to change history. The actual cost of flipping cultures provinces will create its own built in limitation on both the player and the AI. You can't core as many province when you flip culture you can't religiously convert as many provinces when you flip cultures so this actually directly slows down your ability to acquire territory. It also impacts your ability to make a favourable peace because peace treaties cost diplomatic points.
While you are right on many things with regards to culture flipping in EU4, these are not the particular costs - apart from the last one - that truly makes mass culture conversion in non-overseas territory such a remarkably bad idea compared to the alternatives.
Since cores cost AMP and culture costs DMP, your ability to core provinces and flip culture are not directly connected. Likewise, religious conversion does not cost DMP, it costs missionary time. Now, an argument could be made that since culture conversion requires prior coring and religious conversion, these are added costs to culture conversion.
However, consider the following: You will always want to core every single non-overseas conquest without fail unless you are attempting a world conquest,
because overextension sucks. At 100% or below you get annoying modifiers hurting you in many small ways (and big ways if you rely on trade), above 100% it gets nastier. There is absolutely nothing wrong with racking up high overextension during conquest, but I can assure you that players will want to get rid of it rather than live with it, unless they have effectively given up on controlling it. So one way or another, you are going to core your conquests if you think you will be able to hang on to them for a while.
With respects to religious conversion, you will always want to keep every single missionary you have got active all the time so long as you have provinces to convert, because there is
no benefit whatsoever to having a missionary-envoy inactive when he could be active, and there is a great benefit to be had from religious conversion.
So, having argued - for non-overseas territory mind you - that the preconditions for cultural conversion (coring and religious conversion) shouldn't rightly be considered part of the cost of cultural conversion, why is it that I nevertheless agree with you that a person should have his head examined if he goes on a spree of mass cultural conversion non-overseas?
To put it at its most simple, mass conversions of non-overseas territory may seem attractive because it is cheap in ducats (0d - things just don't get cheapter than that!

) and has an immediate reward in getting rid of the non-accepted culture penalties (which, as argued in
my previous post are not actually that bad), but it significantly harms your economy in the long term and limits your opportunities.
What mass culture conversion costs you is the opportunity cost of not spending the DMP on unjustified demands in war (as you mentioned), direct reduction of war exhaustion (seldom a major drain on DMP but a nice option to have available when things go pear-shaped), naval buildings, trade buildings, and some extremely important idea groups. It may even cost you expansion opportunities if you take it to the degree that you culture convert conquests rather than taking Exploration or Expansion when your country is otherwise in a position to significantly benefit from doing so.
Regardless of whether your goal is guns or steel, you gain most bang for the buck in EU4 by building a rock-solid economy and using that to spam the buildings appropriate to your goal everywhere in your realm. Manpower, in particular, so important to a warmonger, is to be found as much in province improvements as it is found in territorial expansion, and the manpower gained from having an economy that can afford to spam the manpower province improvements everywhere dwarfs the gains of culture converting provinces. And I really ought to write a small guide on manpower if nobody else can be bothered.
Of course, if one gets genius diplomatic monarchs time and time again it is possible that one will be able to get everything of the above including tech and have points to spare, in which case cultural conversion is in as a nice DMP dump. Likewise, if one plays a small country that
stays small (only advised for singleplayer

), one can very well run into the surplus DMP situation and decide to culture convert rather than waste points. And finally, if one plays a country that is deliberately crippling its own tech progress in order to fall behind so as to westernize, there might be DMP to spare that cannot be spent otherwise.
But these are all special case and not generally applicable.
For overseas it is simpler - if it is your core and your religion and you do not foresee it being lost in war shortly, culture convert it. With the 80% cost reduction and most overseas provinces having low taxvalues, there really is little reason not to.