(Considering I don't have much knowledge in this area, consider this an outsiders persective.)
‘Retail Compatibility' & That Is So 2005!
With the mods I've seen built and loved by the community (BP, WoD, etc), Retail doesn't seem to be 'needed' in a gaming sense. Retail would end up breaking these mods*1, could you imagine BP or WoD back in 1999?
And it seems that the stable SCP releases we use (3.6.12), are nearly the 'new retail' in some respects as WIP mods want 3.6.12*2 compatibility even though they are being constantly tested with newer nightly builds.
So IMHO here, the newest stable build is pretty much 'retail' in that sense. It is standard, though we would not be here without Retail.
Audience
As previously stated,
I’m willing to bet my hard earned cash that a staggering majority of people are not playing with the retail executable.
, I would also agree.
I do admit however that I switch back to retail every now and then. To see what I first loved, and to see how far FSO has come from the original, the improvements, the gains. I dont stay very long though, maybe just the FS2 campaign and I'll be straight back 3.6.12
The Retail audience, I assume would be small. Most half decent machines should not be running integrated graphics, and should be able to run mediavps fairly well*3 without too many issues. But even then, to run basic mediavps. A major high-tech card is not really needed for that.
Capability
As I said, I see 3.6.12 as being the 'retail' standard. I dont normally see people wanting retail compatibility but 3.6.12*4 compatibility, being completely tested on recent nightlies as well.
Retail seems to be outdated in terms of what can now be done, with the new AI's, sexp's etcetc. While I dont know the full capabilities or compatibilities of these, I assume that they are difficult/impossible in regards to retail behaviour, and would cause quite a few problems*5
Moving Forward
The new code and improvements have pushed FS2 beyond what it could have ever achieved in Retail form, and it still survives and thrives more than 10 years later. Certainly that speaks something here.
But can we maintain compatibility with a game that old?, again IMHO, we will eventually run into the problem of Retail vs Open, and eventually Retail will only hold the community back as it pushes forward.
At the end of the day, I see Retail is Retail and Open/SCP/Mediavps are what everyone*6 wants, compares to, and will improve. The work done here could not have been done without Retail, but we can never change Retail. It always stays the same, and eventually it will not be in a position to help further the community.
*1 - Retail breaking the mod, not the mod breaking retail.
*2 - or the most recent stable build.
*3 - Not stating what graphical/media settings though.
*4 - or the most recent stable build.
*5 - Not just in terms of bugs/errors/crashes. But designs for development of mods/etc.
*6 - I don't speak for everyone, it is my opinion
(Again, I do not have knowledge of FS2 or any SCP code, I am not that knowledgable inregards to FREDding. I have played FS2 for quite a while, and am only a gamer in this sense. And I hope I have provided some sort of outside opinion that is not complete and utter trash).