I've said it multiple times recently, but I actually decided to do just that with 5e, and DID create a document that does just that on DMs guild (called 5e Old school). I think that shows that 5e IS highly malleable and adaptable to other editions and even the older editions right back to the original...but that's my opinion and my take on the matter.
I think what may have happened in regards to the modularity was what I mentioned above, WotC realized that a majority of their audience was not going to come from the older edition players (despite that the numbers of these players outnumber the others at a 4 or 5 to 1 ratio...most are either too preoccupied with other interests these days like family, or will only play the older editions rather than playing a new 5e edition) and instead a majority of those taking the surveys and participating in the playtest were those who were familiar with 3e, PF, and 4e.
That made the focus change and of course, with the players coming from a majority of those disciplines, heavily changed what the emphasis and design ideas were towards the game.
I think it may have still turned out very differently if Monte had not left. In some ways, I think some of the older school ideas in 5e may be directly attributable to some of his contributions.