Surprisingly, the market for RM2K/3 is still pretty strong, mainly because it still offers a great service. One thing about these two engines is that it still has features not offered by default in VX-Ace, such as being able to dictate items to be used by specific party members, which makes it great for designing magic scrolls to be used by a specific character or class.
Plus... there are a crapload of retro sets already available that can't be found for VX or VXA.
This is not to say I fault VXA in any way, because it still has a lot of great features, but that because some of the additional features must be added on with scripts, which you are left to either code yourself or find somebody willing to do it for you, a lot of people will probably continue on using the maker they are most familiar with.
Most of the features for 2k/2k3 have already been scripted and can be found with simple google searching which seems to be foreign to some people because there are so many times that someone can say "use google," the person requesting the content says "I did but I can't find it" and sure enough, the person who said "use google" took the time to type in a few keywords in google and found what the OP was requesting within the top 5 links. Also, those retro sets can be converted simply by figuring out how a tileset is organized and resizing the images and copy-paste. It takes no artistic talent to do this, just an ability to read the help document and download gimp, if you want to go free and legal. I think the reason why some people still using 2k3 is how you describe it that a lot of people continue to use the maker they are most familiar with. There is no issue with that but there are so many similarities between the makers that upgrading is hardly a problem and if you've used the previous ones, it should only take a day or so of toying to figure out that the new one is much better and just as easy to use.
I can't knock the 2k/2k3 users, though. I miss the days when people weren't so addicted to custom systems because custom menus and battles were so cumbersome to make/use and hardly provided decent functionality and that people played the games strictly for story and the reviews were all based on how well the story was done, how well the graphics were put to use and how innovative the battles were while now, you've got more factors that give such ridiculous bonus points to the developer like if the person can script or find someone to script amazing features for them etc. Also, 2k/2k3 had such an awesome style that felt like you were playing an SNES game more often than not, even if it was a poorly made SNES game lol.
Right now, I'm playing my friends game Final Fantasy: Blackmoon Prophecy and it is great. I haven't touched a 2k3 game in years and while it has it's bugs and I'm pointing them out to him as I play it (I do lots of exploring in areas so finding bugs seems to come easy), it is well done and doesn't follow the same bullshit that most FF fan games do where they will throw in so many favorite characters like cloud, sephiroth or whatever. It uses the same naming conventions that the original games used (it's got wedge, biggs, Cid etc. in it) and uses some names of continents frequently heard about in the games, but the design is new and the story is original (except for the whole typical find the crystals blah blah aspect).
I still say, though, if you are going to make this, you should make it for your own personal satisfaction. Not many may use it but some may and you may save them a lot of work. I think you should do whatever you want to do.