People who have mastered EU 10k by playing it competitively for a decade - or years at least - simply won't go and try other options. They will simply leave the game, or become scenario players, or something akin. The maps you are supposedly dueling on lately are presets of the old world map which are not balanced, and no, they are not "new maps".
Competitive players mostly play on EU because it is (sort of) balanced up until ~6 players. The fact that the UK, Germany, Spain, Italy, France, Ukraine and Turkey are being picked all the time is simply a question of efficiency. You do not want to play Montenegro because it is not efficient and is basically a guarantee to lose if your opponent is equally skilled as a Turkey for example.
Likewise, you cannot just go and ask people who mastered gameplay on eu 10k to try out a random competitive south america 25k preset and tell them that it is just as balanced as fun, simply because it's not something that is tried and tested by many, so the plays and the outcomes are largely unknown. In Latin America preset in a 1v1 for example whoever picks Colombia or Venezuela first, basically won the game. This may be "new" for you, only you. It is not new for players who have been playing eu 10k for years.
Therefore, many players did actually play competitively (duel and cw) on other maps in the past. It's simply not happening anymore due to the low amount of players in my opinion.
So to answer your hypothetical question, yes, they will keep playing eu 10k, and when they get bored, they either quit altogether or play something else within AW.
Also... Europe is very dense with a lot of iconic countries, well known, abundant (in many aspects) and a surprisingly small continent. At first, Europe+ did not even exist as a preset, it came later than the regular Eu. It remains very difficult - if not impossible - to have a map that is as good as EU for competitive gameplay;
- matches take mostly ~30 minutes or less, in unique cases ~1hr
- the countries and cities are mostly well known by anyone around the world, meaning it is not that difficult to know what's where, including the fact that you don't expect Serbia to have higher reinf and income than Germany for instance, even without ever seeing the map first
- it contains many separate seas which is good for naval shortcuts and other inherent game mechanisms (NTs are a key concept of aw competitive unless you play GW)
- the map is small, but not too small, just about the perfect size for the default unit ranges and generally how the game works (...) you can take a good look at what is happening and the 4 minute timer will always be optimal
- due to the sheer amount of countries in Europe despite its small size, your expansion options are almost limitless, combined with various strategies that alter unit usage
- it also has this lucky balance where you have 5 countries in the west that are kinda equal in strength when you have 10k funds (ukr vs turk is a little different though)
- etc...
If you find a map that has the same attributes as above, let me know.
Edit:
Perhaps I wasn't clear about one thing, by the way... Playing other presets or other maps for fun is fine, even when they are unbalanced. A competitive scenery HAS TO be balanced, because elo and - in the case of cw - seasonal rewards are at stake. You really don't wanna a play a map and find out turn 3 that the other team will inevitably win because the map is unbalanced. Or things like realizing the map is too big and you need a 5 min timer not 4.
Yes. I guess we will keep doing it in missionary for now.